automatic depth/shadow shaders for arbitrary materials

[robtfm]

Objective

allow depth to be correctly calculated for all materials, including StandardMaterials with AlphaMode::Mask and materials with custom vertex/fragment shaders.

Solution

builds on #5703

• add ShaderSource::Naga variant to support loading shaders from naga directly
• add DepthPipelineBuilder<M: Material> struct which uses naga_oil's prune module to strip the material's shaders down to the minimum required to output @builtin(position) and any discards, then returns a suitable pipeline using the new shaders
• move queue_shadows from light.rs into queue_material_shadows in material.rs, use the depth pipeline builder to generate the pipeline

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note:

• writing a frag shader that uses bindings not available in the ShadowViewGroup (e.g. shadow maps) on the critical path to a discard, will cause the depth pipeline builder to fail
  - using atomics will fail as i haven't really thought about what they mean for pruning yet - do people ever use atomics outside of compute shaders? yes, people use atomics. added an initial prune implementation, not tested though

todo:

• consider making this generic for mesh pipelines / maybe any pipeline (?), not just material pipelines. i guess as long as the specialization key is passed into the builder this would work
• consider an escape hatch so people can author their own depth shaders if required (maybe just push an extra shader_def when deriving the depth shaders)
• try and avoid rebinding materials when they are not required