Blender Git Loki

Clay: Refactor: Port clay to a deferred pipeline.

This means that rendering clay with AO only needs 1 geometry pass.
Thus greatly improving performance of poly heavy scene.

This also fix a self shadow issue in the AO, making low sample count
way better.

We also do not need to blit the depth anymore since we
are doing a fullscreen shading pass.

The constant cost of running the a deferred shading pass is negligeable.
This include quite a bit of code cleanup inside clay_engine.c.

The deferred pipeline is only enabled if at least one material needs it.
Multisampling is not supported yet.

Small hacks when doing deferred:
- We invert the normal before encoding it for precision.
- We put the facing direction into the sign of the mat_id.
- We dither the normal to fight the low bitdepth artifacts of the normal
buffer (which is 8bits per channel to reduce bandwidth usage).