Fix DXF polygon loading by correctly joining oriented segments

[tokoro10g]

This PR fixes the polygon loading algorithm.

For example, if we have

A = [p1, p2]
B = [p3, p2]

it tries to join the segments like the following in l.90:

C = flip(A) + B = [p2, p1, p3, p2]

which is obviously invalid. It should perform C = A + flip(B) instead.

[reilleya]

Hi! Thanks for the contribution. Do you have an example DXF that doesn't load? Took me a moment to think through the existing code, but I believe it is right. In your example, line 90 actually does

C = A + flip(B) = [p1, p2, p2, p3]

because

join = [chunkId, compChunkID, True]

and this block of code reverses the second chunk in the array (not the first) before concatenating them:

addChunk = chunks[join[1]]
if join[2]:
    addChunk.reverse()
chunks[join[0]] += addChunk

Totally possible I'm missing something, though!

[tokoro10g]

Thank you for this awesome project!

Sorry, I was looking at the wrong line, l.93 had an error.

A = [p1, p2]
B = [p1, p3]

It was implemented like

C = A + flip(B) = [p1, p2, p3, p1]

This cannot be solved by simply swapping chunkId and compChunkID in l.93 because the joining order gets also swapped:

B + flip(A) = [p1, p3, p2, p1]

so I decided to add another flag for flipping.