HiresTextures: Support parsing DDS files directly

This leaves DDS textures using DXT1/3/5 compressed in-memory, which can
be passed directly to the backend.

Source/Core/VideoCommon/VideoConfig.cpp

@@ -41,6 +41,7 @@ VideoConfig::VideoConfig()
   backend_info.bSupportsExclusiveFullscreen = false;
   backend_info.bSupportsMultithreading = false;
   backend_info.bSupportsInternalResolutionFrameDumps = false;
+  backend_info.bSupportsST3CTextures = false;
 
   bEnableValidationLayer = false;
   bBackendMultithreading = true;