* must do proper checks for merged ranges when inserting new ones. The checking code assumes that closed ranges are contiguous.
* when discarding parts of overlapping ranges this may not use merged clip values for its checks.
* ranges that have their clip values altered must be released and reinserted to ensure everything is correct.
* let the clipper work on relative angles to simplify the math.
* properly initialize the initial visible range and preserve it for multiple invocations.
* track the maximum visible angular range per sector. While possibly not sufficient to handle every edge case imaginable it has low overhead and is still useful to eliminate obvious cases that do not need more complex checks. It is enough to fix the blue door in Duke E3L4.
* removed unused elements of the clipper.
