cylinder/circle radius modes

adds $rm={INNER_RADIUS,MIDPOINT_RADIUS,OUTER_RADIUS} to cylinder and circle. This adjusts the facets so that the radius is inside the facets, straddling the facets, or outside the facets (the radius lies on the vertexes) (the current current behavior and default behavior).

Adds example025.scad demonstrating this feature, and reference images rendering that as tests.

There may be a glitch in my linux box's OpenGL drivers and the reference images may need to be re-generated to be pixel-perfect.