Support and require Clutter 1.2

- Specify a minimum version of clutter-1.2.0
 - Switch clutter branch in the moduleset to master
 - Replace deprecated cogl_texture/material_unref() with
   cogl_handle_unref()
 - Use cogl_clip_push_rectangle() rather than cogl_clip_push()
 - Replace cogl_check_extension() with strstr - should be
   accurate enough.

https://bugzilla.gnome.org/show_bug.cgi?id=610679

src/shell-slicer.c

@@ -119,7 +119,7 @@ shell_slicer_paint_child (ShellSlicer *self)
 
   cogl_push_matrix ();
 
-  cogl_clip_push (0, 0, width, height);
+  cogl_clip_push_rectangle (0, 0, width, height);
   cogl_translate ((int)(0.5 + x_align_factor * (width - child_width)),
                   (int)(0.5 + y_align_factor * (height - child_height)),
                   0);