In order to move the WindowServer to userspace, I have to eliminate its
dependence on system call facilities. The communication channel with each
client needs to be message-based in both directions.
WSWindowManager::invalidate() had a bug where it would mark the entire screen
rect as dirty, but it wouldn't scheduled a deferred recompose.
This would cause any subsequent calls to invalidate(Rect) to be coalesced
with the pending compose, but the pending compose never happened.
Since we know who's holding the lock, and we're gonna have to yield anyway,
we can just ask the scheduler to donate any remaining ticks to that process.
Work now happens in terms of two messages:
- WM_ClientWantsToPaint
- WM_ClientFinishedPaint
This feels fairly obvious compared to the old Paint/Invalidate.
