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.
Implementation of extra perimeters on steep overhangs/flat unsupported parts
Also contains:
Principal components computation over polygon area
New fast bridge direction estimation which minimizes amount of unanchored bridge endpoints
Adjustment of GUI/3DBed.cpp,hpp to use the more stable triangulation algoritm
derived from SGI glut.
Fix of an extremely slow bridging calculation, caused by an extremely
slow bridged area detection function, of which the results were never used.
Fixes "slicing fails or takes too long #5974"
