Mac drivers are hanging and crashing after large glTexSubImage uploads.
Chunk those uploads to avoid this problematic path in the driver.

Bug: chromium:1181068
Change-Id: I7dca0b8e49420f015ff7830283c14283a0515826
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2928098Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>