Share GraphicsBitmaps between the windowing server and the client process.

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.

Kernel/Syscall.h

@@ -68,6 +68,9 @@
     __ENUMERATE_SYSCALL(sync) \
     __ENUMERATE_SYSCALL(gui_create_window) \
     __ENUMERATE_SYSCALL(gui_destroy_window) \
+    __ENUMERATE_SYSCALL(gui_get_window_backing_store) \
+    __ENUMERATE_SYSCALL(gui_invalidate_window) \
+
 
 namespace Syscall {