Fix line join artifacts with 180º or very sharp angles

[lbud]

This fixes two separate (but in appearance seemingly related) 🐛s with line joins:

• Miter line joins with 180º angles were evaluated to have (NaN, NaN) normal join vectors (e.g. a (1, 0) unit vector and a (-1, 0) unit vector add to (0, 0), whose unit vector is (NaN, NaN)). Because they weren't calculating extrusion vectors correctly these resulted in artifacts like
  
  after:
  

• Miter and bevel line joins with very sharp angles and miter joins are evaluated via the "flipbevel" path: since the angle is so acute, the miter would be far too big, so we bevel it off instead. However, the flipbevel path had a normal vector flipped, so the join's vertices were added in the opposite order expected for indexing and they were creating artifacts such as
  
  (Note: this explanation may not visually make sense, because with very sharp angles we add an extra vertex near the join (something about dashed lines; if you remove that vertex and ⏬ opacity you can see the incorrect triangle indexing:)
  
  after:
  

This also includes some comment spellchecking 😬

I've added two lines to all of the line-join tests in the render suite: one that turns 180º, and one that turns very acutely. These now produce the following results (previously these tests only had the left three lines):

test	master	line-artifacts
bevel
bevel-transparent
default
miter
miter-transparent
round
round-transparent

(Turns out this bug wasn't actually ticketed 😬)

I'd like to get into #3809 / #2316 next (these are more or less the same issue, just applied to different join types), after I wrap up some other outstanding PRs — I dug just enough to know that those are pretty unrelated to ☝️.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality

[lbud]

Currently checking to see whether these affect gl-native too, since I've modified render tests here.

[jfirebaugh]

👌

[lbud]

This does indeed need a gl-native parity PR: ticketed at mapbox/mapbox-gl-native#7778 and added ignores to line-join render tests.

[aaronlidman]

🙇

[lbud]

@aaronlidman sadly though this isn't the same as the line-offset issue you were seeing ☹️ going to hit that after I clear a few outstanding gl-js PRs off my plate.