Bug 1677211 - Add Read access flags to D3D11 staging textures to avoi…

…d slow accesses when reading our intermediate results. r=jrmuizel

Differential Revision: https://phabricator.services.mozilla.com/D97106

gfx/webrender_bindings/RenderCompositorD3D11SWGL.cpp

@@ -225,8 +225,9 @@ bool RenderCompositorD3D11SWGL::MapTile(wr::NativeTileId aId,
   mCompositor->GetDevice()->GetImmediateContext(getter_AddRefs(context));
 
   D3D11_MAPPED_SUBRESOURCE mappedSubresource;
-  DebugOnly<HRESULT> hr = context->Map(mCurrentTile.mStagingTexture, 0,
-                                       D3D11_MAP_WRITE, 0, &mappedSubresource);
+  DebugOnly<HRESULT> hr =
+      context->Map(mCurrentTile.mStagingTexture, 0, D3D11_MAP_READ_WRITE, 0,
+                   &mappedSubresource);
   MOZ_ASSERT(SUCCEEDED(hr));
 
   // aData is expected to contain a pointer to the first pixel within the valid
@@ -309,7 +310,7 @@ void RenderCompositorD3D11SWGL::CreateTile(wr::NativeSurfaceId aId, int32_t aX,
   // add an extra row instead.
   desc.Height += 1;
 
-  desc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;
+  desc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE | D3D11_CPU_ACCESS_READ;
   desc.Usage = D3D11_USAGE_STAGING;
   desc.BindFlags = 0;