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/TextureCacheBase.h

@@ -316,7 +316,8 @@ private:
 
   virtual void CopyEFBToCacheEntry(TCacheEntry* entry, bool is_depth_copy,
                                    const EFBRectangle& src_rect, bool scale_by_half,
-                                   unsigned int cbuf_id, const float* colmat) = 0;
+                                   unsigned int cbuf_id, const float* colmat,
+                                   EFBCopyFormat dst_format, bool is_intensity) = 0;
 
   // Removes and unlinks texture from texture cache and returns it to the pool
   TexAddrCache::iterator InvalidateTexture(TexAddrCache::iterator t_iter);