This CL implements the GL_EXT_compressed_ETC1_RGB8_sub_texture
extension, which was added to relax restrictions on using
ETC1_RGB8_OES for subimage updates.

Test: Temple Run on Android
Bug: b:152512564
Change-Id: I78cfd7dfd54fab36dee59a93b3ec3bfce17e73e4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2123232
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>