Sort out default shader mess

[c42f]

The -shader command line argument is only applied at first startup. This is super screwy and confusing.

A simple fix for now is to obey -shader, except where the user has manually selected one via the GUI. A better fix would be to support per-geometry shaders, though that would need UI work.

[c42f]

(Idea for the shader uniforms UI: support the union of all shader uniforms for currently selected geometry?)

[c42f]

@Ross-Batten here's an issue addressing the issue you just hit around clearplot() starting displaz in a weird state.