Redesign vk_poll_commands to only wait for the completion of some
commands, rather than repeatedly waiting for all of them. Basically, we
will only block once. This makes it much more useful for loops like
“while (!condition) { wait for commands} ”

For the scenario in which we *do* want to wait until all commands have
completed, introduce a helper function “vk_wait_idle” which waits until
the entire device is idle (all commands have been completed).