A boolean was hard-coded to false to disable the use of GL_UNPACK_SKIP_IMAGES
disabled in ImageHelper::stageSubresourceUpdate() (I assume because 3D textures
wasn't yet supported).  The boolean is now set based on type type of the
texture.

Bug: angleproject:3781
Change-Id: I09d9fd142cceb3bebfa341fd0ff9179241c61a9c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1738433Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>