x11: fix memory leaks

Oh, we have to free this stuff. OK.
video: take refresh rate changes into account

This works only on X11, and only if the refresh rate changes due to the
window being moved to another screen (detected by us). It doesn't
include system screen reconfiguration yet.

This calls VOCTRL_GET_DISPLAY_FPS on every frame, which makes me uneasy.
It means extra thread communication with the win32 and Cocoa backends.
On the other hand, a frame doesn't happen _that_ often, and the
communication should still be pretty cheap and fast, so it's probably
ok.

Also needs some extra fuzz for vo_vdpau.c, because that does everything
differently.
video: enable framedropping by default
video: make vo_opengl the default over vo_vdpau

Nvidia's vdpau implementation is pretty good, but other factors make it
much less attractive for use as default VO. For example, Mesa often has
low quality drivers (mess up things with the presentation queue and the
vdpau API time source). Intel ruins things completely, and we're likely
to run on emulation via OpenGL. Compositing has unknown effects (to me
anyway), but appears to reduce the vdpau advantages.

One important reason to prefer vo_vdpau was that it could do proper
framedropping. Framedropping got fixed for the other VOs, so this reason
is going away.