Batch prepasses that use depth only

[tychedelia]

For prepasses that don't need to access material data or write normals/motion vectors, we can batch them together. To do so, we store a new draw command that binds an empty bind group in place of the material bindings and add None for the material id to the batch set key.

Testing:

cargo run --example many_cubes --release -- --shadows --material-texture-count 10

main:

this:

[IceSentry]

I wonder if we could have a more generalized SetEmptyBindGroup, render command. I also needed pretty much the same thing recently.

[IceSentry]

LGTM

I started an example run just to be sure it didn't break anything bevyengine/bevy-example-runner#206

[atlv24]

Nice work

[tychedelia]

The example run looks good so this one should be good to go