Add something like surface shaders to the material system

[pjcozzi]

For example, see http://docs.unity3d.com/Documentation/Components/SL-SurfaceShaders.html

[emackey]

Cozzi mentioned that surface shaders might be a more complete solution to issues I've come across while developing a grid material to go with the Ellipsoid primitive. Two issues of note are:

• Need to specify an axis to be the "pole" of the grid, where the lines come together. Currently hard-coded to Z axis, but spacecraft uncertainty volumes typically put the pole on the X axis (the general direction of travel).
• Would like the grid material to be aware of things like the distance from the camera to the primitive (center or surface) as opposed to distance from camera to fragment, so that the grid could better choose its own resolution without help from the primitive, possibly with lines that gently fade in or out as the camera moves.

The above are possible use-cases for surface shaders, and will be implemented by other temporary means until surface shaders exist.

[mramato]

Is this issue still relevant given geometry and appearances?

[pjcozzi]

Nah. This is covered by appearances and their roadmap, #766.