Hide SHAPE_LINE_CHAIN accessors
SHAPE_LINE_CHAIN forms the basis of our polygon and PNS LINE support. This needs to be extended to support non-linear tracks. The approach I am proposing here is to prevent modification of the underlying point-based structure. This will facilitate adding additional structures to SHAPE_LINE_CHAIN while providing a unified interface to modify the chain.
These commits make all direct point access CONST.
Existing routines that read points are converted to CONST reads.
Existing routines that modify points are converted to equivalent move/rotate/flip/del-insert routines.