Document or provide a way to produce stereoscopic overlays

[vilhalmer]

Pulling this out of #155 (comment). It would be very nice to provide a drop-in scene for producing 3D overlays using the side-by-side texture flags, or at the very least a wiki page explaining how to do it.

[bradragon]

I'll see about getting stereo working in my other non-Godot project. That should help inform what does and doesn't work.

[bradragon]

Well, I tested using this image, using SetOverlayFlag to SideBySide_Parallel as per here and it worked great. I just used SetTexture() to load the image into the texture.

So the missing gap is a camera that will render SBS textures.