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.

LibC/stdlib.h

@@ -19,5 +19,9 @@ void qsort(void* base, size_t nmemb, size_t size, int (*compar)(const void *, co
 void exit(int status) __NORETURN;
 void abort() __NORETURN;
 
+#define RAND_MAX 32767
+int rand();
+void srand(unsigned seed);
+
 __END_DECLS