This is pretty cool. :^)
GraphicsBitmaps are now mapped into both the server and the client address
space (usually at different addresses but that doesn't matter.)
Added a GUI syscall for getting a window's backing store, and another one
for invalidating a window so that the server redraws it.
Userspace programs can now open /dev/gui_events and read a stream of GUI_Event
structs one at a time.
I was stuck on a stupid problem where we'd reenter Scheduler::yield() due to
having one of the has_data_available_for_reading() implementations using locks.
