capture/replay: implement CaptureGetTexImageANGLE_pixels Bug: angleproject:5646 Change-Id: Ia4ae0fc76eae4f9c8c133b0501eb792e794c4cfc Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2718172 Commit-Queue: Gert Wollny <gert.wollny@collabora.com> Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>