Don't make MIDI regions in note mode completely invisible.

This makes for extremely confusing behaviour, particularly when creating new
regions.  This is still probably too transparent, IMO, but at least isn't
totally mysterious.  "visible" is a bit of a misnomer in the code, now.
Fix error when create-dragging note backwards past start of region (#5934).
Fix wonky note length when create-dragging notes backwards.

Specifically, when pivoting from forwards to backwards (around the drag start
point), the note length was too long.  Setting both the start and end x
coordinates of the rect every time to the right value does the right thing.