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/Software/SWmain.cpp

@@ -73,6 +73,7 @@ void VideoSoftware::InitBackendInfo()
   g_Config.backend_info.bSupportsBPTCTextures = false;
   g_Config.backend_info.bSupportsCopyToVram = false;
   g_Config.backend_info.bForceCopyToRam = true;
+  g_Config.backend_info.bSupportsFramebufferFetch = false;
 
   // aamodes
   g_Config.backend_info.AAModes = {1};