Advanced note tail rendering

[robbertzzz]

Added support for spline-based note tails for slides (see image), both for pitched slides and unpitched slides (image is unpitched, i.e. it doesn't have a definite end pitch and dies out). Also added support for sine wave-based vibrato note tails, with guaranteed vertical alignment at the start and end of the note tail. Note tails are meshes that are constructed mathematically; slides are based on different beziér curves and vibrato is a sine-wave that automatically fits the best number of waves over the length of the tail with a minimum of one full sine wave.

[darthLeviN]

I don't see any issues. I ran it and got some nice trails that start from one note and end at another. No errors either. Seems good.