VideoCommon: Add usedualsrc field to BlendingState

This is because we re-use BlendingState for our internal drawing (e.g.
RasterFont) and for these shaders, we can't assume the presence of a
second color output.

Source/Core/VideoBackends/OGL/Render.cpp

@@ -1214,7 +1214,7 @@ void Renderer::SetBlendMode(bool forceUpdate)
   state.Generate(bpmem);
 
   bool useDualSource =
-      g_ActiveConfig.backend_info.bSupportsDualSourceBlend &&
+      state.usedualsrc && g_ActiveConfig.backend_info.bSupportsDualSourceBlend &&
       (!DriverDetails::HasBug(DriverDetails::BUG_BROKEN_DUAL_SOURCE_BLENDING) || state.dstalpha);
 
   const GLenum src_factors[8] = {

Source/Core/VideoCommon/RenderState.cpp

@@ -65,6 +65,7 @@ void BlendingState::Generate(const BPMemory& bp)
   colorupdate = bp.blendmode.colorupdate && alpha_test_may_success;
   alphaupdate = bp.blendmode.alphaupdate && target_has_alpha && alpha_test_may_success;
   dstalpha = bp.dstalpha.enable && alphaupdate;
+  usedualsrc = true;
 
   // The subtract bit has the highest priority
   if (bp.blendmode.subtract)

Source/Core/VideoCommon/RenderState.h

@@ -26,6 +26,7 @@ union BlendingState
   BitField<14, 3, BlendMode::BlendFactor> dstfactoralpha;
   BitField<17, 3, BlendMode::BlendFactor> srcfactoralpha;
   BitField<20, 4, BlendMode::LogicOp> logicmode;
+  BitField<24, 1, u32> usedualsrc;
 
   u32 hex;
 };