Initial attempt to give desktop ETC/EAC formats support.

Bug: angleproject:3676
Change-Id: Id147b0c1808e30df77097d0c7ff6a06534554b93
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1699307
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>