By default enable swap throttling

This ensures that when we initialize a CoglOnscreenTemplate that
->swap_throttled is set to TRUE so by default Cogl applications should
have their swap buffer requests throttled to the vblank frequency.

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

(cherry picked from commit bdac9ab56284fb84ec09af176c7e31c44dc1bfc6)

cogl/cogl-onscreen-template.c

@@ -57,6 +57,7 @@ cogl_onscreen_template_new (CoglSwapChain *swap_chain)
   else
     onscreen_template->config.swap_chain = cogl_swap_chain_new ();
 
+  onscreen_template->config.swap_throttled = TRUE;
   onscreen_template->config.need_stencil = TRUE;
   onscreen_template->config.samples_per_pixel = 0;