GL_OES_texture_cube_map_array is core in 3.2. This CL adds the
necessary validation for the frontend, as well as support for this
extension on the GL backend. The next step is to add the changes
to the translator.

Bug: angleproject:3584
Change-Id: I751a9c9f71a553b05fdf6673250290806d8cfbff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2215306
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>