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/VideoBackends/OGL/main.cpp

@@ -140,6 +140,7 @@ static void InitBackendInfo()
 	g_Config.backend_info.bSupportsGeometryShaders = true;
 	g_Config.backend_info.bSupports3DVision = false;
 	g_Config.backend_info.bSupportsPostProcessing = true;
+	g_Config.backend_info.bSupportsPaletteConversion = false;
 
 	g_Config.backend_info.Adapters.clear();