Revisit geometry selection

[c42f]

The current selection model has a few "interesting" attributes:

• Objects which are invisible (eg due to a discard in the shader) are selectable.
• Line segments and triangles are selectable via their vertices only.
• It's based on a non-projected distance, so lidar noise near to the camera position can get selected by mistake.

Selection of objects based on the actual OpenGL framebuffer around the mouse position would help with some of these things. The issue is that the user can do arbitrary things to the shader; this is great sometimes, but means we can't get object IDs flowing through the shader easily. The depth buffer might be good enough, though some heuristics will probably be required - needs experimentation!

[c42f]

@Ross-Batten here's an issue which may help with selecting lines. Can't promise when I'll get to it though, it's not super simple.