libweston: set the presentation clock in the compositor
Let backends declare the presentation clocks they can use with a new bitfield weston_backend::supported_presentation_clocks
and set presentation clock after loading the backend in the compositor.
Merge weston_compositor_set_presentation_clock_software()
and weston_compositor_set_presentation_clock()
into one function and export it to the compositor.
In the future, this can be extended to determine the subset of clocks supported by all backends.
This is meant as an alternative to !913 (closed).