Hi.
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Guesst looked into the whole anything-else-than-triangles thing ages ago over on WC3C.
We established that there was only one other (implemented/working) option: trianglestrip, and this turned out only to be able to render a single strip (so we surmised it was used for ribbon emitters).
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Well, well, well, after a billion years, I finally tested and it appeared you were wrong ! Things like lines can totally be used and I'm sure those are not the only ones.
You can punch that Guesst guy for me the next time you see him