Only the read framebuffer should affect the flipping for ReadPixels.

BUG=angleproject:2673

Change-Id: I3733bb823f5f3ebd55df63a36cc99b510dc0b1aa
Reviewed-on: https://chromium-review.googlesource.com/1138817Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>