Split weston_compositor_init into base and _gl

Split weston_compositor_init into base and _gl

weston_compositor_init is always called late because most
implementations can't initialise GL until fairly late in the game.
Split it into a base version with the same name, followed by
weston_compositor_init_gl which can be called later on.

This simplifies compositor-wayland, which no longer needs a separate
global handler just for wl_seat.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>