osd: minor simplification of vo_osd_changed()

vo_osd_changed() was a weird function: it was used both to query and
mutate state, which is a bad combination. The VOs used it to query
and reset the state, and the mplayer frontend mostly used it to set
the state. In some cases, the frontend did both (that code used a
variable "int hack" to backup the state and set it again).

Simplify it and make the VOs use a vo_osd_has_changed() function to
query whether the OSD bitmaps have to be recreated. vo_osd_changed()
on the other hand is now used to update state only. The OSD change
state is reset when osd_draw_text() is called.

Update vo_corevideo.m to use vo_osd_resized() as well (forgotten change
from libass-OSD merge).

Simplify osd_set_text() and its usages.

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