Implement dual-source blending in shader

For some GLES drivers that don't support dual-source blending, but do
support GL_EXT_shader_framebuffer_fetch, this might be useful.

Source/Core/VideoBackends/D3D/main.cpp

@@ -82,6 +82,7 @@ void VideoBackend::InitBackendInfo()
   g_Config.backend_info.bSupportsBitfield = false;
   g_Config.backend_info.bSupportsDynamicSamplerIndexing = false;
   g_Config.backend_info.bSupportsBPTCTextures = false;
+  g_Config.backend_info.bSupportsFramebufferFetch = false;
 
   IDXGIFactory2* factory;
   IDXGIAdapter* ad;