Fixed some issues in internal anchors of the Adaptive Cubic infill.
The ugly and dangerous implicit casting operators in Line, MultiPoint,
Polyline and Polygon were made explicit.
Tighter parsing of PrusaSlicer's own G-code annotations
to avoid clashes with comments inside user G-codes.
Also the GCodeReader was extended to return string_views instead
of copying a substring, and the GCodeProcessor was partially adapted
to string_views.
Iterator loops replaced with C++11 loops.
Fixed clone() methods to return an ExtrusionEntity*.
PerimeterGenerator now uses move semantics on ExtrusionEntity a little
bit more.
