Traces: Fix screenshot pixel pack state Without this fix, if the trace had non-default pixel pack state, our screenshots were broken. This just backs up the current state, sets defaults, then restores them. Test: return_to_monkey_island screenshots Bug: b/383027719 Change-Id: I8eaa1d58cdf34fbbaefdf4bbb054bff3fd8a5e9b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6079310 Reviewed-by: Roman Lavrov <romanl@google.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>