Store egl::Image size/format information at initialization time. This avoids potential null-dereferences on the source if it is orphaned. BUG=angleproject:2668 Change-Id: I5d591a941d114bb231044572a31a8c43cf3a9c4f Reviewed-on: https://chromium-review.googlesource.com/1155104 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>