scene.update_mesh_positions() misdocumented as computing normals

[LohitPetikam]

Documentation error where update_mesh_positions is said to compute normals from the new vertex positions, which is not the case.

[errollw]

+1 - also have this issue. Would love to use MeshUpdate for the space-savings but can't unless the normals are correct.

[errollw]

I think I have a fix for this: replacing VectorBufferNone with base_mesh->compute_normals(positions, base_mesh->triangles()) on line 410 of scene.cpp.

However, I can't seem to make a branch so I can make a PR for this fix. Any suggestions? Also just happy if someone with the right permissions can make this fix.

[tjcashman]

@errollw you can create a fork with your branch, and then create a pull request to merge from your fork. Here's an example of what that looks like: #38