Add fixed orientation mobjects to opengl

[sudgy]

As far as I can tell, there's no way to have fixed orientation mobjects (i.e. a mobject that is positioned in 3D space but is not rotated in 3D space) when using OpenGL, which you can do in cairo (using add_fixed_orientation_mobjects). To be clear, this is different than fixed in frame mobjects, which OpenGL does support. This is a major showstopper for me in using OpenGL for 3D scenes. I'm honestly shocked that I haven't heard anybody talk about this at all, as it seems like an essential feature for 3D scenes, but I asked on Discord and people seemed to suggest that it was impossible, so I'm saying something here.