Widely spaced, beamed notes cause incorrect beam slopes

[Silverwolf90]

These beams are seen in the tradeoffs test in tests/beam_tests.js

Here are some beams formatted to a 300px width:

Here are the same beams formatted to a 500px width:

The first beam became very slanted (although not incorrect, just ugly). The second beam's slope is going in the wrong direction.

Originally discovered in PR https://github.com/0xfe/vexflow/pull/456

[0xfe]

Interesting. The slope I think is an easy fix, but I'm surprised about the change in direction.

[rvilarl]

@AaronDavidNewman could you look into that? The problem is still there