D3D11 FL9_3, like Shader Model 2.0 in D3D9, doesn't support reading from SV_Position in the pixel shader. We have to reconstruct gl_FragCoord using dx_ViewCoords instead.

Change-Id: I7e898038d210d73a9d224dcc18b033e5cd4a56f5
Reviewed-on: https://chromium-review.googlesource.com/234277Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>