[d3d11] Only store low 8 bits of stencil reference

Seems to match behaviour of the D3D11 runtime, in that OMGetDepthStencilState
will not retain the high bits.

Found by CME. Should fix doitsujin#1784.

src/d3d11/d3d11_context.cpp

@@ -2525,7 +2525,11 @@ namespace dxvk {
       m_state.om.dsState = depthStencilState;
       ApplyDepthStencilState();
     }
-
+
+    // The D3D11 runtime only appears to store the low 8 bits,
+    // and some games rely on this behaviour. Do the same here.
+    StencilRef &= 0xFF;
+
     if (m_state.om.stencilRef != StencilRef) {
       m_state.om.stencilRef = StencilRef;
       ApplyStencilRef();