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.