Ensure up-to-dateness of global_sub information upon request.

Some demuxers (like mpg) can detect subtitles only while playing,
which is the reason why MPlayer always - indirectly - calls
update_global_sub_size() prior to dealing with subtitles.

Functions mpctx_get_global_sub_size() and mpctx_get_global_sub_pos(),
however, merely return last determined values, thus missing information
that may have been arisen after MPlayer has checked last for subtitles
(when playback started).

Merge the two mpctx_get_global_sub_*() functions into one for easier
handling.

Retrieve global_sub pos by a mp_property_do("sub") command which
performs the necessary update of both global_sub variables, pos and
size.

Reported by Lode Leroy, lode.leroy gmail com.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37255 b3059339-0415-0410-9bf9-f77b7e298cf2