VideoCommon: Start putting common texture decoding code in TextureDecoder_Common

This pulls all the duplicate code from TextureDecoder_Generic /
TextureDecoder_x64 out and puts it in a common file. Out custom font
used for debugging the texture cache is also pulled out and put in a
common "sfont.inc" file. At some point we should also combine this font
with the other six binary fonts we ship.

Source/Core/VideoCommon/CMakeLists.txt

@@ -23,6 +23,7 @@ set(SRCS	BPFunctions.cpp
 			Statistics.cpp
 			TextureCacheBase.cpp
 			TextureConversionShader.cpp
+			TextureDecoder_Common.cpp
 			VertexLoader.cpp
 			VertexLoaderManager.cpp
 			VertexLoader_Color.cpp