Started GLSL snippet library

Started a library of common GLSL functions in `vispy.visuals.shaders.snippets` (is that too deep?).

Every Python module defines constants with strings of GLSL that can be readily used with the shader composition system in `vispy.visuals.shaders` (including template variables). There is extensive documentation for each snippet (input, output, template variables).

The current PR only implements a single GLSL function, but many more could be added later.

Closes #162.