Lazy shader compilation

[simontaylor81]

Instead of compiling shaders as soon as the script asks for one, just return a handle and defer until later.

Advantages:

• Allows rest of script to execute, revealing errors, etc.
• Necessary for parallel shader compile.
• Possibly could only compile shaders actually needed for rendering (e.g. if different permutations are required depending on user variables).

[simontaylor81]

Done in 110da9b. Not going to do compilation of only used shaders because it gets messy with user properties -- what if the used subset changes frame-to-frame?