Adds a new extension to allow transcode ETC formats to suitable BC formats.
This commit implements ETC1 to BC1. More formats will be supported soon.

BUG=angleproject:1285

Change-Id: Iacbfbc2248dfe1aebf24b92696249a9404e331cd
Reviewed-on: https://chromium-review.googlesource.com/316511Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>