macosx: Fix resume playback in certain situations
macosx: Fix resume playback in certain situations

For resumption we need to rely on the "time" variable instead of
"position", as the total duration of the file might not have been
set yet.

Also simplifies the code.

close #11981

xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc