3D viewer: move helper functions from 3d_draw.cpp to 3d_draw_helper_functions.cpp. Add comments in .h sources. Better management of background color. coding style fixes.

dialog_track_via_size: set minimal size  and make dialog resizable.
Update demo file.

   File "rules" has instructional text as comments near top.

2) Convert all text files in repo to LF line ending form.
   Any checkout done with "rules" in play will convert the working
   tree to native line ending, while keeping repo as LF line ending.

AUTHORS.txt: add some contributor names (I certainly forgot some other contributors, and I apologize).
Demos: convert .brd files to .kicad_pcb files and fix 2 broken files