Minor comments that can be solved by another PR:

• It would be great if the patches you applied are documented in the thirdparty/dxflib/VERSION file or in a separate thirdparty/dxflib/PATCHES.md file.
• Redundant guard in tessellation

  int steps = (int)(fabs(angle2 - angle1) / 2.0) + 1;
  if (steps < 1)
      steps = 1;

The + 1 already guarantees steps >= 1, so the if is dead code. Harmless, but worth removing.

Else great idea with external thirdparty directory. I will merge this.

Originally posted by @spe-ciellt in #296 (comment)