xwayland: add workaround for drivers that don't support impicit sync

Without either implicit or explicit synchronization, the result of rendering is pretty much undefined, and many glitches can appear. This still doesn't synchronize buffer release, but it works around most glitches until explicit sync is supported.

Signed-off-by: Xaver Hugl <xaver.hugl@kde.org>

I would've liked for this not to be needed, but !967 (merged) seems to still take a little bit more time, and this one is small and self-contained enough so that it can be backported to older Xwayland versions as a bugfix too.

@ekurzinger I made the first commit authored by you, because it's exclusively bits and pieces copied from !967 (merged); is that okay with you?