After validation, the enums are simply forwarded to the native
drivers. The BPTC formats are supported on both OpenGL and D3D.

The included test coverage covers the API quite well, but only has
basic coverage for correct decoding of texture data. More coverage for
texture data could be added later.

BUG=angleproject:2869
TEST=angle_end2end_tests

Change-Id: I3de37972dcf13c6fa3fc1bc429a2627523a4a082
Reviewed-on: https://chromium-review.googlesource.com/c/1261675
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>