Add unit test for polyline_lines
Remove sporadic duplicate useless lines. #772
Disable randomize starting points by default
New option to enforce support material on a given number of layers to improve adhesion of objects with poor footprint
Bugfix: avoid-crossing-perimeters was failing on layers only containing single walls. #978
Rename shortest_path to chained_path, which reflects its algorithm more correctly
shortest_path() in MotionPlanner was actually correctly named shortest_path()
Optimize cycling in get_layer_range()
Rename shortest_path() in tests too. #980
Skip support material for one-layer objects
Refactor chained_path(), add chained_path_items() and chained_path_points()
Fix for Replicator 1 not finishing builds.

If the progress reaches 100% before last layer, the firmware erroneously stops reading the file.
Bugfix: angle threshold was considered even when enforcing support
New option to only infill where strictly needed for supporting ceilings
New simpler and more robust implementation of infill every 'n' layers
regression test for combined infill #834

Guards against issue of slightly off-vertical walls triggering
extra infill on layers that should be empty when combining infill.
account for extrusion width when clearing layers for combined infill

Otherwise you get extraneous fill up against sloped walls in regions
of layers that should not have fill, like in #834.
avoid overlap of single layer and multilayer fill

A single-layer rectilinear or honeycomb fill under a low slope
wall/ceiling or extra perimeter will stick out from under the
overhang, because those fills get extended beyond their fill_surface
boundaries to purposely overlap perimeters. This causes interference
with multil-layer fills. This commit clips the single layer fill
boundaries a bit more where they interface with multi-layer fill
boundaries, to avoid overlap.
added file combineinfill.t
Disable extra perimeters if fill density is zero as they wouldn't be supported by anything and would just float inside the object. infill_only_where_needed should be the method of choice for hollow objects