Hide emulated ETC2/EAC formats from WebGL contexts In addition to not exposing the dedicated ANGLE-specific extension name, do not add these formats to the internal formats table. Bug: angleproject:7601 Change-Id: I2e3fdd99129823b8092bfa35cb95023ae5008edd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3853591 Auto-Submit: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>