This is the 2nd payload of GL_ANGLE_lossy_etc_decode feature. In this change,
RGB8, SRGB8, RGB8A1, and SRGB8A1 formats in ETC2 family can be converted
to BC1.

BUG=angleproject:1285

Change-Id: I96fe2f07c62716a31d37f20a202b6cabbb4ebbd2
Reviewed-on: https://chromium-review.googlesource.com/447846
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>