Add helper for getting shader pass type

[slimbuck]

This PR adds a helper function for categorising shader passes into pass types and uses them when building shaders.

This fixes invalid shaders in editor when rendering custom passes (specifically pass 24 used for outline rendering).

This following function is added:

ShaderPass.getType(shaderPass)

Which returns one of the following new constants:

const SHADERTYPE_FORWARD = 'forward';
const SHADERTYPE_DEPTH = 'depth';
const SHADERTYPE_PICK = 'pick';
const SHADERTYPE_SHADOW = 'shadow';

[mvaligursky]

approving, few comments need fixing.