Fix up screenshot saving for trace tests. Now works when run in a sequence. Also saves RGB images to avoid issues with the alpha being inconsistent and also flips images vertically to fix the rendering. Bug: angleproject:4615 Change-Id: I8d3b38c5d914e0ca2227320ac42a0e28acd12c4d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2187971 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>