Join two curves with a fillet arc near a given point.

Input parameters:

1. Ca → First curve to fillet.
2. Cb → Second curve to fillet.
3. Rd → Fillet radius interval. A fillet arc with the biggest possible radius within this range will be created.
4. Pt → Optional cue point for fillet location.

Output parameters:

1. Cr → Filleted and joined curves.
2. Rd → Radius used for fillet.