Disallow GLES BGRA render buffers.
Disallow GLES BGRA render buffers.

This stops Mesa from complaining when using GLES in the back end.

BUG=angleproject:1301

Change-Id: I0b8cb78efea97c1ea0d21cd6b9f656e53b9b6cc2
Reviewed-on: https://chromium-review.googlesource.com/331395
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Frank Henigman <4f3ccfa167f952ff7263448ff4377acff6a23e5c@chromium.org>

MSOpenTech/angle,ecoal95/angle,ppy/angle,mikolalysenko/angle,MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,mikolalysenko/angle,ppy/angle,ppy/angle,mikolalysenko/angle,domokit/waterfall,mikolalysenko/angle,ecoal95/angle,ppy/angle,domokit/waterfall,MSOpenTech/angle,ecoal95/angle