Make font loading use mmap().

This exposed a serious race condition in page_in_from_inode().
Reordered the logic and added a paging lock to VMObject.
Now, only one process can page in from a VMObject at a time.
There are definitely ways to optimize this, for instance by making
the locking be per-page instead. It's not something that I'm going
to worry about right now though.

SharedGraphics/Font.h

@@ -12,6 +12,8 @@ public:
 
     RetainPtr<Font> clone() const;
 
+    static RetainPtr<Font> load_from_memory(const byte*);
+
 #ifdef USERLAND
     static RetainPtr<Font> load_from_file(const String& path);
     bool write_to_file(const String& path);