RFC: vkr: add async roundtrip support

Closed Yiwei Zhang requested to merge zzyiwei/virglrenderer:async-trip into master

This changes the roundtrip waiting from driver side to the renderer side. However, that requires to unlock/relock the ctx mutex with a cond to allow renderer roundtrip to arrive while the ring is waiting. Would like to send this working prototype out for advices on better structuring first.

/cc @olv @ryanneph

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