Add tests for readPixels, copyTexSubImage2D and copyTexImage2D which
attempt to read outside the framebuffer boundary.  readPixels and
copyTexSubImage2D should not touch destination pixels which correspond
to pixels outside the framebuffer.  copyTexImage2D should write (0,0,0,0)
to those pixels.

BUG=angleproject:1815

Change-Id: I94042b5b08e319f28cb97d13613fe52514b67213
Reviewed-on: https://chromium-review.googlesource.com/452728
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>