RS274X Gerber files using only regions (polygons) can be valid, and are not old RS274D files
Now a warning is raised only if there are some missing D codes definitions
(RS274D file or broken RS274X file)
Remove unused var and add comments
Fixes: lp:1850821
https://bugs.launchpad.net/kicad/+bug/1850821
The standard DIM() macro was not typesafe as it happily deferred errors
to runtime that can be caught at compile time. Replacing it with a
generic C++11 constexpr allows for typecasting, comparison and compile
time error checking.
Previously, the M and G00, G01, G02 and G03 commands were read, but nothing was actually done.
The current support is a bit rough, but it allows reading some drill files with routing commands.