Define Sweep Gradient angle conditions

[DJMcNab]

We need to define what happens if:

• the end angle is less than the start angle
• they're equal
• when they subtend more than $$2\pi$$

The obvious first step is to catalogue what Vello Classic does in these cases.

This issue does not block a release; these questions aren't really blocking anything.

Originally posted by @DJMcNab in #130 (comment)