properly define miter_limit values

[mikebmcl]

It needs a definition and right now doesn't have one. Essentially, greater than 0 degrees, not 180 degrees, and less than 360 degrees. Any value that would cause sin(val) to be 0.0f. Then change the interface to use degrees or radians or both (I'm pretty sure both was the LEWG consensus in Kona or Toronto for pi vs. theta so that will be relevant here I think).