Refactored PerimeterGenerator for functional style, better constness
with the goal of calling PerimeterGenerator::process_xxx()
for each surface at once to collect its fill expolygons.
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.
