onscreen: Adds support for resizable windows

This adds api to be able to request that the window system allows a
given onscreen framebuffer to be resizable, and api to add and remove
resize handlers to be called whenever the framebuffer does actually
change size.

The new functions are:
  cogl_onscreen_{get,set}_resizable()
  cogl_onscreen_{add,remove}_resize_handler()

The examples cogl-hello and cogl-x11-foreign have been updated to use
the new api. To smoke test how Cogl updates the viewport automatically
in response to window resizes the cogl-hello test doesn't explicitly
respond to resize events by setting the viewport and cogl-x11-foreign
responds by setting a viewport that is offset by a quarter of the
window's width/height and half the width and height of the window.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit a1a8cc00bfa2cecaf1007aec5f3dd95dc07b1786)

cogl/cogl-onscreen-private.h

@@ -46,6 +46,19 @@ struct _CoglSwapBuffersNotifyEntry
   unsigned int id;
 };
 
+typedef struct _CoglResizeNotifyEntry CoglResizeNotifyEntry;
+
+COGL_TAILQ_HEAD (CoglResizeNotifyList, CoglResizeNotifyEntry);
+
+struct _CoglResizeNotifyEntry
+{
+  COGL_TAILQ_ENTRY (CoglResizeNotifyEntry) list_node;
+
+  CoglOnscreenResizeCallback callback;
+  void *user_data;
+  unsigned int id;
+};
+
 struct _CoglOnscreen
 {
   CoglFramebuffer  _parent;
@@ -64,6 +77,9 @@ struct _CoglOnscreen
 
   CoglSwapBuffersNotifyList swap_callbacks;
 
+  CoglBool resizable;
+  CoglResizeNotifyList resize_callbacks;
+
   void *winsys;
 };
 
@@ -77,4 +93,7 @@ _cogl_framebuffer_winsys_update_size (CoglFramebuffer *framebuffer,
 void
 _cogl_onscreen_notify_swap_buffers (CoglOnscreen *onscreen);
 
+void
+_cogl_onscreen_notify_resize (CoglOnscreen *onscreen);
+
 #endif /* __COGL_ONSCREEN_PRIVATE_H */