Add preprocessor for ShaderLab compiler

[Sway007]

The precessor should be capable of the following task:

• support #include and #define macro expansion

  take the function-like macro expansion into account:

  #define saturate( a ) clamp( a, 0.0, 1.0 )

  edge cases and issues processing should be coherent with the glsl 100 specification
• can interpret the expression of branch macro, e,g. #if, ifdef, ifndef, else, elif, endif

  #if defined(XXX) && YYY > 1.0 || ZZZ == 2
     ......
  #elif ...
    ......
  #endif

• extract macros which have impact on the generated code (for engine use)

  not necessarily implemented in preprocessor, maybe can be done with another more light way