Gives invisible stems point extents. (issue 7762049)

[k-ohara5a5a]

So then the convention for an extent is
'(+inf.0 . -inf.0) is empty, but for skylines in the cross direction it
is an infinite wall;
'(0 . 0) is a point, but ignored for skylines;
'(-inf.0 . +inf.0) is an infinite wall all the time.

Could you filter out this effect on RhythmicStaves ?

My heavy-metal band uses a flame-shaped glyph for some Stems, and it
needed zero X-extent for some spacing reason.  While fixing another MIDI
bug I had the Midi_performer listen for Stems and send a message, if on
channel 10, when it sees a zero-width Stem, by convention.  That message
fires the flamethrowers, so if you push this change I have to remind the
boys to avoid that part of the stage in case the drummer has any beamed
rests.

https://codereview.appspot.com/7762049/