VideoBackends: Use VideoCommon shader generators for efb2tex copies.

This will generate one shader per copy format. For now, it is the same
shader with the colmat hard coded. So it should already improve the GPU
performance a bit, but a rewrite of the shader generator is suggested.

Half of the patch is done by linkmauve1:
VideoCommon: Reorganise the shader writes.

Source/Core/VideoCommon/CMakeLists.txt

@@ -39,6 +39,7 @@ set(SRCS
   TextureCacheBase.cpp
   TextureConfig.cpp
   TextureConversionShader.cpp
+  TextureConverterShaderGen.cpp
   TextureDecoder_Common.cpp
   VertexLoader.cpp
   VertexLoaderBase.cpp