Drag/Drop work-around midi note bleeding

Drag/Drop work-around midi note bleeding

During DnD, the region uses the 'old/current'
midi_stream_view()'s range and its position/height calculation.

Ideally DnD would decouple the midi_stream_view() for the
region(s) being dragged and set it to the target's range
(or in case of the drop-zone, FullRange).
but I don't see how this can be done without major rework.

For now, just prevent visual bleeding of events in case
the target-track is smaller.