Vulkan: Add support for ETC1 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>