libobs-d3d11: Fix gs_enable_color

It looks like the link between the gs layer rgba enable flags and the
underlying D3D states never got fully implemented.

This change adds the missing piece, fixing an issue I had in a plugin
wherein I couldn't write a blended value to a RGBA render target without
also changing the alpha of the dest pixel.  Debugging that led to the
missing gs_enable_color functionality.

Closes jp9000/obs-studio#1064

libobs-d3d11/d3d11-subsystem.cpp

@@ -341,7 +341,10 @@ ID3D11BlendState *gs_device::AddBlendState()
 		bd.RenderTarget[i].DestBlendAlpha =
 			ConvertGSBlendType(blendState.destFactorA);
 		bd.RenderTarget[i].RenderTargetWriteMask =
-			D3D11_COLOR_WRITE_ENABLE_ALL;
+			(blendState.redEnabled   ? D3D11_COLOR_WRITE_ENABLE_RED   : 0) |
+			(blendState.greenEnabled ? D3D11_COLOR_WRITE_ENABLE_GREEN : 0) |
+			(blendState.blueEnabled  ? D3D11_COLOR_WRITE_ENABLE_BLUE  : 0) |
+			(blendState.alphaEnabled ? D3D11_COLOR_WRITE_ENABLE_ALPHA : 0) ;
 	}
 
 	SavedBlendState savedState(blendState, bd);