wgl: Call st_swapbuffers instead of st_notify_swapbuffers.

wgl: Call st_swapbuffers instead of st_notify_swapbuffers.

This will get single buffer, double buffer, and
joint single/double buffer (typical in CAD applications) done right,
at least as far as the frambuffer is concerned.

There are still problems with multiple contexts using the same
framebuffer because st_framebuffer_* calls assume the framebuffer
is bound to a single context.