Sync all framebuffer attachments when checking completeness. If the backend requires that a framebuffer is synchronized before checking backend completeness, make sure all attachments are synchronized too. GL has completeness rules based on GL_BASE_LEVEL and GL_MAX_LEVEL of texture attachments which are not syncrhonized until the textures are. If they are left un-sychronized during completeness checks, the driver will tell us that the framebuffer isn't complete. Bug: chromium:1455725 Change-Id: I7c3bf6a38f63feaa863f4d8914c3655e286dd768 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4678286 Reviewed-by: Brian Ho <hob@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>