Vulkan: Fix the debugger screenshot in headless

There's no swapchain, so reading the backbuffer asserted. Fail the
readback, and have gpu.buffer.screenshot fall back to the displayed
framebuffer.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

Common/GPU/Vulkan/thin3d_vulkan.cpp

@@ -1891,6 +1891,10 @@ bool VKContext::BlitFramebuffer(Framebuffer *srcfb, int srcX1, int srcY1, int sr
 
 bool VKContext::CopyFramebufferToMemory(Framebuffer *srcfb, Aspect aspects, int x, int y, int w, int h, Draw::DataFormat format, void *pixels, int pixelStride, ReadbackMode mode, const char *tag) {
 	VKFramebuffer *src = (VKFramebuffer *)srcfb;
+	if (!src && vulkan_->GetSwapchain() == VK_NULL_HANDLE) {
+		// No backbuffer to read, as in headless.
+		return false;
+	}
 
 	int aspectMask = 0;
 	if (aspects & Aspect::COLOR_BIT) aspectMask |= VK_IMAGE_ASPECT_COLOR_BIT;

Core/Debugger/WebSocket/GPUBufferSubscriber.cpp

@@ -278,7 +278,12 @@ static void GenericStreamBuffer(DebuggerRequest &req, std::function<bool(const G
 void WebSocketGPUBufferScreenshot(DebuggerRequest &req) {
 	GenericStreamBuffer(req, [](const GPUDebugBuffer *&buf, bool *isFramebuffer) {
 		*isFramebuffer = false;
-		return GPUStepping::GPU_GetOutputFramebuffer(buf);
+		if (GPUStepping::GPU_GetOutputFramebuffer(buf)) {
+			return true;
+		}
+		// Without a backbuffer (headless Vulkan), the displayed PSP framebuffer is the closest thing.
+		*isFramebuffer = true;
+		return GPUStepping::GPU_GetCurrentFramebuffer(buf, GPU_DBG_FRAMEBUF_DISPLAY);
 	});
 }