manually merged LibreCAD2 changes

[lordofbikes]

This is the result of syncing independent changes to libdxfrw from LibreCAD 2 and LibreCAD 3.
It brings more stability for reading DWG files, but there is still work to do.
We should take care to keep this repo and the LibreCAD_2 embedded code in sync now.

@feragon, before merging this branch, we have to check the LibreCAD_3 implementation for 2 new abstract methods in the interface.

libdxfrw/src/drw_interface.h

Line 191 in c420c48

virtual void addPlotSettings(const DRW_PlotSettings *data) = 0;

libdxfrw/src/drw_interface.h

Line 202 in c420c48

virtual void writeObjects() = 0;