PBR Materials

[tychedelia]

Adds support for PBR materials based on Bevy's StandardMaterial.

At a high level, our material API is going to operate on named fields: mat.set("foo", 1.0). This is the intended API for custom materials and largely mirrors how Processing4 handles its Shader type. Because StandardMaterial is a built in, in this PR we handle it as a special case where we map field names to setting properties on the StandardMaterial asset struct. But, from the perspective of the user, we want a pbr material to basically feel like any other material, just one whose shader you don't control.

Some more specific notes:

• We support both "immediate" and "retained" materials. Immediate materials will allow the user to change the pbr properties of their current batch/rendering context. Retained materials are an entity that can be used to set the "current" material for following geometry draws. We may want to change this and have the geometry draw just also accept a material param.
• We may want to think more about how this interacts with 2d. In theory, these methods only make sense in 3d, but in practice there's nothing wrong with users setting these, they just will look weird unless they use lights, which will also look weird in ortho perspective.
• The batching logic has gotten a bit more tricky.