15623b26b9
Custom normals are interpolated with the new Manifold boolean code. In the other two solvers, there is no interplation for the CD_PROP_INT16_2D type so custom normals were left at (0,0) when an exact copy from the input corner isn't possible. The new code for manifold first does a join mesh, which converts custom normals to float3, which can be (and are) interpolated. However, if the input face (or face fragment) has flipped normal in the boolean output, then the custom normal should be flipped to. The fix is to do that for attributes called "custom_normal". The general case of a "normal-like" thing with a different name remains unsolved, but I doubt that case comes up often.
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