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.

Make weston_compositor_set_presentation_clock() internal and replace
weston_compositor_set_presentation_clock_software() with an exported
weston_compositor_backends_loaded(), which is called by the compositor
after the backend is loaded.

In the future, this can be extended to determine the subset of clocks
supported by all backends.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>