Canvas ignores RenderSystem.runAsFancy()

[grondag]

This is useful for mods that want to render entities and block entities to GUI.

Probably the best way to handle is to expose a hook that forces target to main frame buffer irrespective of materials and then activate/deactivate that hook from RenderSystem.runAsFancy().

This mod has a test case: https://github.com/wisp-forest/affinity/

See discord Canvas channel for additional detail.