* AC: Support Double-Sided Faces
The AC format marks double-sided SURF elements with the 0x20 flag, which Assimp ignored. This commit adds support for double-sided faces.
On encountering a double-sided face via the flag mentioned above, the front face is generated as usual but is then duplicated. The winding order of the duplicate is flipped to form a back face. Vertices are duplicated too so that back faces work correctly with normal vector generation and face smoothing.
* Add test file
* Simplify test case
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Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>
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