Fixes the multiple reader issue #6541 properly without resorting to a
linear search kludge.
All the read state has been pulled out into a MidiCursor which the
caller is required to pass. The playlist keeps cursors for all the
regions it is reading, any number of cursors are allowed at a time.
MidiCursor should probably be made a smarter and more fool-proof
object (and/or possibly merged with some of the other tracker/fixer
stuff) but for now I wanted to keep it simple.