The ETC2 format is backwards compatible with ETC1.  ETC1 RGB8 is thus
supported by implicitly using the ETC2 RGB8 Vulkan format.

Bug: angleproject:2670
Change-Id: I59a112da5ed3a0b90f90a8b4c350c94d43b31120
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520987
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>