WindowServer: Factor out compositing from WSWindowManager into WSCompositor.

This is far from finished and the two classes are awkwardly grabbing at each
other's innards, but here's a first step in the right direction.

Servers/WindowServer/main.cpp

@@ -1,6 +1,7 @@
 #include <WindowServer/WSScreen.h>
 #include <WindowServer/WSWindowManager.h>
 #include <WindowServer/WSEventLoop.h>
+#include <WindowServer/WSCompositor.h>
 #include <signal.h>
 #include <stdio.h>
 
@@ -18,6 +19,7 @@ int main(int, char**)
 
     WSEventLoop loop;
     WSScreen screen(1024, 768);
+    WSCompositor::the();
     WSWindowManager window_manager;
 
     dbgprintf("Entering WindowServer main loop.\n");