Add TextureReplacer unit test; make replacer testable standalone

Add unittest/TestTextureReplacer which creates a fictive texture pack
(textures.ini with readable invented hashes plus real PNG files), loads
it via the replacer, and verifies lookups, filtering, hashranges, mip
levels, and missing/ignored entries.

To make the replacer runnable outside the emulator:
- The constructor now accepts a null DrawContext (formats just default
  to unsupported).
- FindReplacement/FindFiltering use the replaceEnabled_ member instead
  of the global config.
- Added TextureReplacer::LoadPackForTesting() to load an ini from a
  path directly.

CMakeLists.txt

@@ -1391,6 +1391,7 @@ if(UNITTEST)
 		unittest/TestVFS.cpp
 		unittest/TestZipSlip.cpp
 		unittest/TestLzrc.cpp
+		unittest/TestTextureReplacer.cpp
 		unittest/TestRiscVEmitter.cpp
 		unittest/TestLoongArch64Emitter.cpp
 		unittest/TestSoftwareGPUJit.cpp