This adds cogl_onscreen_template_set_swap_throttled() api that allows
developers to specify their preference for swap buffer throttling
up-front as part of the onscreen template that is used to create a
CoglDisplay when initializing Cogl. This is desirable because some
platforms may not support configuring swap throttling on a per
framebuffer basis and also since applications often want to apply the
same policy to all onscreen framebuffers anyway.