Add helper class for a rendered cursor

Add a magical cursor framebuffer class for handling when you
want to render the cursor on the server side. Keeps the cursor
specific magic in one contained place.
diff --git a/common/rfb/VNCServerST.h b/common/rfb/VNCServerST.h
index ed7833a..ede5223 100644
--- a/common/rfb/VNCServerST.h
+++ b/common/rfb/VNCServerST.h
@@ -224,9 +224,7 @@
 
     Point cursorPos;
     Cursor cursor;
-    Point cursorTL() { return cursorPos.subtract(cursor.hotspot); }
-    Point renderedCursorTL;
-    ManagedPixelBuffer renderedCursor;
+    RenderedCursor renderedCursor;
     bool renderedCursorInvalid;
 
     // - Check how many of the clients are authenticated.