Ignore baseline-skip in the triangle markup command

The \triangle markup command produces glyphs that are too large at large font sizes, and too small at small font sizes. It also causes LilyPond to abort when the baseline-skip value is 0 - producing no PDF but leaving behind temporary files.

Remove baseline-skip from the calculation of the triangle and update the existing multiplier to a value that produces triangles that scale more proportionately with font size. This also circumvents the problem with LilyPond aborting. Add a reg test.

Closes #6096 (closed)