Improve flags_r11 setting in preR13 encoding

[michal-josef-spacek]

I am going to improve flags_r11 setting. I created PR https://github.com/LibreDWG/libredwg/pull/757 which:

• Describe these numbers precisely
• Added flags_r11 from the structure if is defined

One thing is TODO, which I am not able to set now:

• Add dwg->header.section[id].flags_r11 = __FROM_CONTROL__ in https://github.com/LibreDWG/libredwg/blob/master/src/in_json.c#L3264 It helps conversions from JSON to DWG and better ability to test conversion from DWG to JSON and back.

[rurban]

This comment TODO: maybe add a missing CONTROL object here. GH #453 is something else. We do store all the CONTROL objects plus its flags_r11 in the json already.

But we need your new function for indxf

[michal-josef-spacek]

@rurban In JSON, yes, but conversion from JSON to DWG is not working (e.g. in the case of flags_r11)