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.
15623b26b9