D3D11: Query for output merger logic op support and use logic op code only if supported

Previously code assumed that if DX11.1 runtime is supported, logic ops will,
but Windows 7 SP1 with a Platform Update supports DX11.1 runtime without logic ops.
This created pretty jarring visual artifacts, which now should be gone OR replaced
with much less jarring errors.

Source/Core/VideoBackends/D3D/D3DBase.h

@@ -43,6 +43,9 @@ std::vector<u32> GetAAModes(u32 adapter_index);
 // Checks for support of the given texture format.
 bool SupportsTextureFormat(DXGI_FORMAT format);
 
+// Checks for logic op support.
+bool SupportsLogicOp(u32 adapter_index);
+
 }  // namespace D3D
 
 }  // namespace DX11