This approach properly handles outside-render-pass clears.

Bug: b/190622922
Change-Id: Ia4a9d6ec13d7da8c4a445af1127e82c03f37e8b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2951960Reviewed-by: Mark Lobodzinski <mark@lunarg.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>