Prior to this change, the render area was decided when the render pass
was started, and remained fixed.  If a small scissor was initially used,
this created a render pass with a small area.  If then the scissor
region was expanded, the render pass was broken.

This change instead expands the render area on scissor change to avoid
breaking the render pass.  If glInvalidateSubFramebuffer previously
successfully resulted in storeOp=DONT_CARE, this optimization may need
to undo that.  As a result, the invalidate area is stored in the render
pass and if the render area grows beyond that, invalidate is undone.

Bug: angleproject:4988
Change-Id: I4e8039dec53a95a193a97cb40db3f71e397568d6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2508983
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>