Fixes for involute_gear.scad

1. Revert adding of two_d option -- does the exact same thing as flat.
2. Fix handling of the rounding of the root circle (cut the 2d shape before extrusion instead of the 3d shape to avoid breaking helical and herringbone gears)
3. Add internal option for generating internal gears (clearance and backlash are adapted for this purpose). Using a negative backlash on the previous version works, but not the clearance, because we need to expand the addendum for internal gears instead of shrinking the dedendum.