vulkan: Don't wait until graphics are done computing

vulkan: Don't wait until graphics are done computing

We can let the GPU do its stuff without waiting. The GPU knows what it's
doing.

Which means we now get a lot of time to spend on doing CPU things (read:
we're way better in benchmarks).

The old behavior is safer, so we want to keep it around for debugging.
It can be reenabled with GSK_RENDERING_MODE=sync.