Make GraphicsBitmaps be Region-backed when running in the kernel.

This is a lot better than having them in kmalloc memory. I'm gonna need
a way to keep track of which process owns which bitmap eventually,
maybe through some sort of resource keying system. We'll see.

Kernel/MemoryManager.h

@@ -11,6 +11,8 @@
 #include <AK/AKString.h>
 #include <VirtualFileSystem/VirtualFileSystem.h>
 
+#define PAGE_ROUND_UP(x) ((((dword)(x)) + PAGE_SIZE-1) & (~(PAGE_SIZE-1)))
+
 class Process;
 extern Process* current;