- Expose GLES 3.0 feature of 2_10_10_10_REV texture type
  on GLES 2.0 as EXT.
- Handle alpha channel value as 1.0 when used with RGB format.
- Add test for "RGB+UNSIGNED_INT_2_10_10_10_REV" case into TextureUploadFormatTest.

BUG=angleproject:3232.
Test:
 dEQP-GLES2.capability.extensions.uncompressed_texture_formats.GL_EXT_texture_type_2_10_10_10_REV
 dEQP-GLES2.functional.fbo.completeness.renderable.texture.*2_10_10_10_rev
 dEQP-GLES3.functional.fbo.completeness.renderable.texture.*2_10_10_10_rev
 KHR-GLES2.core.internalformat.*2_10_10_10_rev*
 KHR-GLES3.core.internalformat.*2_10_10_10_rev*

Change-Id: Iac00517971f9242161115c7256117a69093fb5df
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1732618
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>