1) Flipped the order of "discover_vertical_shells" and "process_external_surfaces",
now the external surfaces are expanded after "discover_vertical_shells"
aka "ensure vertical wall thickness" is solved.
2) Reworked LayerRegion::process_external_surfaces() to only expand into
"ensure vertical wall thickness" regions, also the expansion is done
in small steps to avoid overflowing into neighbor regions.
also:
Utility functions reserve_more(), reserve_power_of_2(), reserve_more_power_of_2()
Various SurfaceCollecion::filter_xxx() modified to accept an initializer list of surface types.
New bridges detector refactored to accept overhang boundaries.
BoundingBoxWrapper was moved from RetractCrossingPerimeters to AABBTreeIndirect.
into a graph with links to the layer above / below.
In addition:
Members of LayerRegion were made private, public interface const only.
this->m_xxx replaced with just m_xxx
SurfacesPtr was made a vector of const pointers.
