Fix initialization for ETC2 punchthrough alpha formats * Zero-filled ETC2_A1 data must be treated as opaque black * Removed init functions when emulation is not needed * Fixed initialization for emulated ETC2_R8G8B8A1_SRGB_BLOCK Bug: angleproject:6936 Bug: angleproject:6941 Change-Id: Ie5555bbaa6b1a2bcfd9c22cb9c17d1a96e72aa70 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3406761 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>