Technically we could just grab directly at the CharacterDevices like we've
been doing but I'd like to stay away from in-kernel behavior like that.
This should eventually become a userspace process.
Windows that don't intersect any of the dirty rects don't need to be copied
into the back buffer since they won't be affected by the subsequent flushes.
It's a bit confusing that the "current" process is not actually running
while we're inside the scheduler. Perhaps the scheduler should redirect
"current" to its own dummy Process. I'm not sure.
Regardless, this patch improves responsiveness by allowing the scheduler
to unblock a process right after it calls select() in case it already has
a pending wakeup request.
The system can finally idle without burning CPU. :^)
There are some issues with scheduling making the mouse cursor sloppy
and unresponsive that need to be dealt with.
