BUG=angleproject:1002

Changed since last failed 16-bit patch:
- Disabled usage of 16-bit DXGI textures on all Intel cards
- Added DXGI->GL conversion entries for 16-bit formats, which
  fixes FBO readback issues. The ES CTS and dEQP tests which
  failed with the last patch now pass.

Change-Id: Ib66abf73f23dbb4329f0a32d7f3dafbbc23e6c08
Reviewed-on: https://chromium-review.googlesource.com/273712Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>