Shader texture bias and ao fixes

[slimbuck]

This PR makes the following changes to the standard material:

• declare global textureBias uniform in non-main pass (this was causing shader compile errors in shadows etc)
• split ambient occlusion into 'get' and 'apply' to match most other components. This change is made with a view to splitting out material front and back end.

Chunk changes:

• ao.frag.js:
  • rename function from applyAo to getAo
  • function only calculates and sets the global dAo, doesn't also apply the resulting value to diffuse
• added aoDiffuseOcc.frag.js
  • applies the calculated global dAo to the diffuse component