VideoBackends: Clear uid bits that are unused for the current backend

Currently, this is only the logic op bit, but this will be extended to
the framebuffer fetch/blend modes. In the future, when/if we move to
VideoCommon pipelines, this state will be part of the pipeline UID
anyway, and we can mask it out in the backend by using a two-level map,
so the shaders/programs are shared.

Source/Core/VideoCommon/UberShaderPixel.h

@@ -29,4 +29,5 @@ ShaderCode GenPixelShader(APIType ApiType, const ShaderHostConfig& host_config,
                           const pixel_ubershader_uid_data* uid_data);
 
 void EnumeratePixelShaderUids(const std::function<void(const PixelShaderUid&)>& callback);
+void ClearUnusedPixelShaderUidBits(APIType ApiType, PixelShaderUid* uid);
 }