Decode EFB copies used as paletted textures.

A number of games make an EFB copy in I4/I8 format, then use it as a
texture in C4/C8 format.  Detect when this happens, and decode the copy on
the GPU using the specified palette.

This has a few advantages: it allows using EFB2Tex for a few more games,
it, it preserves the resolution of scaled EFB copies, and it's probably a
bit faster.

D3D only at the moment, but porting to OpenGL should be straightforward..

Source/Core/VideoCommon/VideoConfig.h

@@ -159,6 +159,7 @@ struct VideoConfig final
 		bool bSupportsBBox;
 		bool bSupportsGSInstancing; // Needed by GeometryShaderGen, so must stay in VideoCommon
 		bool bSupportsPostProcessing;
+		bool bSupportsPaletteConversion;
 	} backend_info;
 
 	// Utility