Adds fallback for some RGB textures using the corresponding RGBA formats
and modifies fallback calculation to not require filtering/rendering for
formats which are not required to support filtering by GL spec.

Bug: angleproject:3190
Bug: angleproject:3196
Change-Id: I7beaf9881d63e3c6bd9339faede0333919a4174c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1665894
Commit-Queue: James Dong <dongja@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>