Vulkan: remove libretro's global function-pointer wrapper hack

libretro/libretro_vulkan.cpp got PPSSPP's libretro core working with
RetroArch's Vulkan integration by globally monkey-patching PPSSPP's Vulkan
loader function pointers (vkCreateInstance, vkCreateDevice,
vkCreateSwapchainKHR, vkAcquireNextImageKHR, vkQueuePresentKHR,
vkQueueSubmit, etc.) so the unmodified VulkanContext class would end up
wrapping RetroArch's already-existing VkInstance/VkDevice instead of
creating its own, and so a fake VkSwapchainKHR (a self-managed array of
images synced against RetroArch's retro_hw_render_interface_vulkan
callbacks) could stand in for the real swapchain that libretro's Vulkan
model doesn't have. Flagged in-code as "a wacky wrapper".

Replaces that with first-class support in VulkanContext for the two things
libretro actually needs:

- Adopting an externally-created instance/device instead of faking
  vkCreateInstance/vkCreateDevice: VulkanContext::CreateInstanceExternal()
  adopts RetroArch's VkInstance; CreateDevice() gained optional
  extraDeviceExtensions/extraRequiredFeatures params so RetroArch's
  requirements get merged into a real vkCreateDevice() call;
  ownsInstance_/ownsDevice_ flags (the latter set via
  SetDeviceExternallyOwned()) mean DestroyInstance()/DestroyDevice() skip
  the real vkDestroy* calls when something else owns the object, without
  needing to intercept anything. VulkanLoader gained
  VulkanLoadFromGetInstanceProcAddr() for bootstrapping from a
  host-supplied proc-addr getter instead of dlopen/dlsym-ing the loader
  ourselves - vkGetDeviceProcAddr is resolved via the real instance handle
  (not NULL), since per the Vulkan spec it's not one of the handful of
  commands queryable with a NULL instance.
- A pluggable presentation backend (Common/GPU/Vulkan/VulkanPresentation.h)
  for hosts with no real VK_KHR_swapchain, replacing the fake-swapchain-
  handle trick. VulkanContext::GetPresentation() is null by default, so
  every existing platform's real-swapchain code path is untouched;
  libretro/LibretroVulkanPresentation implements this interface directly
  against retro_hw_render_interface_vulkan, as real class state instead of
  file-scope globals. Several pieces of state that are normally only
  populated as a side effect of ReinitSurface()/InitSwapchain() - the
  graphics queue/queue family index (ChooseQueue() is entangled with
  real-surface presentation-support checks), the swapchain format, and
  the available present modes - needed presentation-aware fallbacks since
  libretro never calls that real-surface path at all.

libretro/LibretroVulkanContext.cpp now drives VulkanContext's real, public
API directly - no more hijacked function pointers, no more fake surface or
swapchain. libretro/libretro_vulkan.cpp is deleted.

Verified with a full build+run in RetroArch (not just compile-time
checks): the libretro Makefile doesn't track header dependencies
(cl.exe doesn't support -MMD/-MP, and Makefile.common never sets up an
equivalent), so a `make clean` full rebuild is required after any header
change to avoid linking stale object code from before the change - several
of the fixes above were initially masked by exactly that.

Windows/PPSSPP.vcxproj

@@ -529,7 +529,7 @@
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">true</ExcludedFromBuild>
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|x64'">true</ExcludedFromBuild>
     </ClCompile>
-    <ClCompile Include="..\libretro\libretro_vulkan.cpp">
+    <ClCompile Include="..\libretro\LibretroVulkanPresentation.cpp">
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">true</ExcludedFromBuild>
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">true</ExcludedFromBuild>
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'">true</ExcludedFromBuild>
@@ -882,6 +882,14 @@
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">true</ExcludedFromBuild>
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|x64'">true</ExcludedFromBuild>
     </ClInclude>
+    <ClInclude Include="..\libretro\LibretroVulkanPresentation.h">
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">true</ExcludedFromBuild>
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">true</ExcludedFromBuild>
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'">true</ExcludedFromBuild>
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|ARM64'">true</ExcludedFromBuild>
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">true</ExcludedFromBuild>
+      <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|x64'">true</ExcludedFromBuild>
+    </ClInclude>
     <ClInclude Include="..\libretro\libretro_core_options.h">
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">true</ExcludedFromBuild>
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">true</ExcludedFromBuild>

Windows/PPSSPP.vcxproj.filters

@@ -202,7 +202,7 @@
     <ClCompile Include="..\libretro\libretro.cpp">
       <Filter>Other Platforms\libretro</Filter>
     </ClCompile>
-    <ClCompile Include="..\libretro\libretro_vulkan.cpp">
+    <ClCompile Include="..\libretro\LibretroVulkanPresentation.cpp">
       <Filter>Other Platforms\libretro</Filter>
     </ClCompile>
     <ClCompile Include="..\libretro\LibretroD3D11Context.cpp">
@@ -433,6 +433,9 @@
     <ClInclude Include="..\libretro\LibretroVulkanContext.h">
       <Filter>Other Platforms\libretro</Filter>
     </ClInclude>
+    <ClInclude Include="..\libretro\LibretroVulkanPresentation.h">
+      <Filter>Other Platforms\libretro</Filter>
+    </ClInclude>
     <ClInclude Include="..\libretro\libretro.h">
       <Filter>Other Platforms\libretro</Filter>
     </ClInclude>