Material compiler v2

[sunho]

Description

Very rough draft that shows new IR interface with POC insert_subdag, dead_strip, subexpression_elimination implementation.

It introduces "Value" abstraction that tries to solve multiple issues:

• Now don't have to maintain "children" array even if the node will not ever have children list
• Proper hashing of "normal mapped" subtree; by having normal "Value" inside all bsdf node if bsdf uses different normal, the hash of subtree will be different
• Arbitrary subexpression elimination can remove unnecessary texture prefetch or normal precomputation without need of separate pass (just typical subexpression elimination will remove it since hash is same)
• Open up possibility of supporting arbitrary expression (not major point but nice to have)

TODO list:

• Make code compile
• Testing infrastructure
• Logging support
• Port mitsuaba frontend
• Implement VM backend (possibly in HLSL)