Support shared glsl libraries

[xian]

Maybe something like an #include or #import directive?

Some considerations:

• Are imported functions namespaced/namespaceable? A library could easily have its own privately-used functions like random() that ideally wouldn't be exposed.
• How do you provide libraries, and where do they live?
• • maybe Sparkle Motion ships with some, like rgb2hsv()
• • maybe you can create (more)
• (More...)