This marks the internal format pixel bytes compute helpers and the
Context scratch and zero buffer helpers as "no discard". This prevents
sketchy code where we don't catch error values.

Bug: angleproject:2713
Change-Id: Ic6d285897dc36ad529cd5361d2763338578df61f
Reviewed-on: https://chromium-review.googlesource.com/1134207Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>