Kernel: Make Process a Weakable class.

Use this to fix a use-after-free in ~GraphicsBitmap(). We'd hit this when
the WindowServer was doing a deferred destruction of a WSWindow whose
backing store referred to a now-reaped Process.

SharedGraphics/GraphicsBitmap.h

@@ -43,7 +43,7 @@ private:
     size_t m_pitch { 0 };
 
 #ifdef KERNEL
-    Process* m_client_process { nullptr };
+    WeakPtr<Process> m_client_process;
     Region* m_client_region { nullptr };
     Region* m_server_region { nullptr };
 #endif