cogl: Add debugging environment option COGL_SYNC

When set it forces a cogl_framebuffer_finish on every frame to ensure the GPU and CPU stay synchronous. This is needed when benchmarking so you can see how long the GPU is really taking to complete each frame. As mentioned already in the docs for cogl_framebuffer_finish.

Without this flag the GPU will complete its work in the background, so your measured render times would be artificially low. In that case you only know there's a bottleneck when your swap events arrive late and you're decimated to a low frame rate.