Commit FS#10605 - stable playback on low memory swcodec targets by Matthias Schneider...

Commit FS#10605 - stable playback on low memory swcodec targets by Matthias Schneider.  Should allow stable playback on targets with less then 4MB of RAM and sofware decoding such as the Sandisk Clip, c200v2, m200v4 and probably others.  Fixes several problems in buffering that occured when the files to be buffered weren't much smaller then the ring buffer size.  Fixes a bug where move_handle would corrupt the audio buffer when trying to copy a handle that both wrapped around the highest address in the ring buffer and overlapped part of the source and desination ranges.  Moves the decision in playback.c about when to update the current track handle from audio_check_new_track to after the metadata has been buffered.  Corrects several other minor pieces of code.  I've logged about 100 hours without a crash on various players with this patch but its possible it breaks some combination of players and features I haven't thought to test.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@23680 a1c6a512-1295-4272-9138-f99709370657