Shader API proposal

[Jacky Guo]

This is my proposal as to how the shader API should work:

In javafx.scene.paint 2 new classes would be added:

   - Shader
   - ShadedMaterial

ShadedMaterial would contain methods relating to the material itself,
probably including texture maps and the like.

Shader would contain more low-level functions, like passing values to the
shader or similar.

I'm not sure if all the functionality should belong to one class, but in my
mind, separating it like this will have its benefits.

In addition, I haven't dabbled much with native graphics, so I will be
needing some guidance on that end.