Fix level set shading

[MarkGillespie]

I found two minor bugs in polyscope's marching tets implementation.

1. Emitted faces were oriented using u_sliceVector rather than the gradient of the scalar function. This works fine for slicing along planes, but for arbitrary scalar functions u_sliceVector is completely independent from the function
2. Normals weren't transformed to screen space

I fixed these bugs and enabled level set visualization in the GUI

[nmwsharp]

Awesome, thank you! I didn't actually pull this down to test, but it all looks sane to me.

[nmwsharp]

Do you have an image/gif of this feature that you could easily share here? If it's enabled in the GUI now I should probably add it to the docs, and an image would be nice :)