Should be applying a 16-bit mask instead of 8-bit.

Bug: angleproject:3926
Change-Id: I2b0a28a5de42437890a9c0d1bbc57b48a5d64451
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1814980Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>