DXF: add creation option to set and DXF system variables

[rouault]

Fixes #11423

[rouault]

@atlight I was wondering if using the CRS unit as a default for INSUNITS would be a sane default ? And what about MEASUREMENT (should it be related to CRS units?) ? For now, I've kept the values from header.dxf (inches / imperial) as the default

[coveralls]

coverage: 74.175% (+0.002%) from 74.173%
when pulling 0d98f32 on rouault:dxf_INSUNITS_MEASUREMENT
into 7d69cae on OSGeo:master.

[atlight]

Haven't tested, but looks good to me. Just minor stuff here.

[atlight]

@atlight I was wondering if using the CRS unit as a default for INSUNITS would be a sane default ? And what about MEASUREMENT (should it be related to CRS units?) ? For now, I've kept the values from header.dxf (inches / imperial) as the default

(Reading your comment after leaving my review) I think this makes a lot of sense. If your CRS is in metres you ideally should get a DXF in metres without taking any active steps to ensure that.

Not really sure about MEASUREMENT, it doesn't seem very important. It's not something my company or our clients ever use.

[rouault]

I think this makes a lot of sense. If your CRS is in metres you ideally should get a DXF in metres without taking any active steps to ensure that.

ok, I've remove the INSUNIT=FROM_CRS mode, and rather use a default INSUNIT=AUTO mode that tries to use the CRS info when available, and fallbacks to the value of the template header.dxf file