FunctiongetMiterOffset

getMiterOffset(prevSegment, nextSegment, widthOffset): vec3
Shifts vertices so that we can join corners of line segments w/ miter
Parameters
- prevSegment: vec3
  direction of previous vertex
- nextSegment: vec3
  direction of next vertex
- widthOffset: number
Returns vec3
miterOffset - add or subtract this from a vertex to get offset needed