GL: Align emulated ETC1 handling with other backends Use emulatedEtc1 limitation to hide it from WebGL. Bug: chromium:1048244 Change-Id: Id2fd6b5d934f3caeb455e8f0fc198445e8c00084 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2803136 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>