fix #35171: allow backwards lines when possible

Backwards lines are now drawn if there is no text or if the line is diagonal.

In the process, I fixed issues with the bbox calculation for textlines with diagonal lines that I discovered while fixing this.  I also needed to fix http://musescore.org/en/node/35186 - pedal lines always having a start hook on read - in order to update the vtest.  I'll close that issue manually when then is merged.