Revert "Make ScheduleSwap block on vblank"

This reverts commit 3b84975509c5cc522e6b12eda6e889c066b347e2.
This was bad - blocking the whole server.

Better approach is to patch the X server, make it delay the SwapBuffers
reply until the buffers have been swapped. In the mean time, it can
continue servicing other clients.
https://gist.github.com/dsd/f84efae45862b1ed48e4