allow script to set OSD resolution

mp.set_osd_ass() now takes width and height as first two parameters.
These set the libass OSD resolution.

Also add mp.get_screen_resolution(), which returns the window size as
seen by the OSD rendering code. In some cases, this window is actually
"stretched" (consider anamorphic video with -vo xv or anything with
-monitoraspect). So we possibly need to compensate for that.