We don't always need to end the render pass when updating the scissor,
since it will be ended later when necessary.

This change is in preparation for optimizing resolving multisample
images with glBlit, since the render pass needs to be updated before
it's ended.

Bug: angleproject:4753
Test: CQ
Change-Id: Ie657587ca9f4461dcc03f0f9c251ac2c17398f5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2327334Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>