Only allow TexStorage2DEXT if we expose the extension

If the underlying driver doesn't support the necessary extensions, TexStorageImpl crashes.
Also, make sure we enable the extension on es3 command buffers (and thus, possibly disable BGRA if it is not compatible).

BUG=None
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2000923002
Cr-Commit-Position: refs/heads/master@{#395466}
5 files changed