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/TextureCache.h

@@ -48,6 +48,7 @@ private:
 	~TextureCache();
 
 	TCacheEntryBase* CreateTexture(const TCacheEntryConfig& config) override;
+	void ConvertTexture(TCacheEntryBase* entry, TCacheEntryBase* unconverted, void* palette, TlutFormat format) override;
 
 	void CompileShaders() override;
 	void DeleteShaders() override;