Fix FS#9110 and its maybe-dupes.
The issue happened when the value of 'filling' was STATE_FINISHED: the low_buffer_callback wouldn't do anything even when there would still be data remaining to be buffered, leading to a shortage of audio data. Only making the callback act even when filling is STATE_FINISHED (the simple fix) isn't right because of cases when the last track in the playlist is fully buffered. Therefore I added a new state to distinguish between when the last track is fully buffered (STATE_FINISHED) and when it isn't (STATE_END_OF_PLAYLIST).
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@17875 a1c6a512-1295-4272-9138-f99709370657