The following scenario was mishandled:

- MSRTT draw with an unresolve operation (i.e. has two subpasses)
- Deferred clear
- Flush deferred clear with MSRTT framebuffer not needing unresolve
  (i.e. has one subpass)

Bug: chromium:1178693
Change-Id: If3548e99897d698d61dfafbe9f86193723d06e5a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2697648
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>