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>