Improve ogr dxf driver spline entity reading (ticket #6436)

[perob]

Changed source code and added tests with data.

Read full definition of spline entity (degree, control points, weights and knots) from dxf files and create spline. If only degree and control points found in dxf file then recalculate knots. Weights are always 1 if not present in dxf file.

[rouault]

The Travis build errors with :

  TEST: ogr_dxf_30 ... OGRFeature(entities):1
  Layer (String) = 0
  SubClasses (String) = AcDbEntity:AcDbSpline
  ExtendedEntity (String) = (null)
  Linetype (String) = (null)
  EntityHandle (String) = 24C
  Text (String) = (null)
  LINESTRING (0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,14 2)

fail
    line 123: Error in vertex 0, off by 14.

[perob]

Ah, thanks.
I created test files manually and obviously made some mistakes which passed through sanity checks.

I will add some more sanity checks.

[perob]

That is it. Ready for commit.

[rouault]

Merged in r33888. Thanks