Add a PrepareBatches render set that runs after PrepareResources.

[pcwalton]

Objective

PR #10231 needs to add some logic that runs as part of PrepareResources but before batch building, because it creates data that batch building needs (the lightmap index, which is part of the Mesh structure).

Solution

This PR adds a new render set for the batching operations. It was split out of #10231 at reviewer request.

[superdump]

Hmm, I’ll need to look more closely at this. It’s not obvious if it’s necessary / useful to add another render set at a glance. It isn’t harmful for sure and maybe it is indeed useful for specifying dependencies more simply for your case.

[pcwalton]

@superdump Look at the interaction between https://github.com/bevyengine/bevy/pull/10231/files#diff-cbd1eecdb5d976e19535f4a9afc4d7cf545c4e4648c58eadecc1880f8304929aR160-R202 and https://github.com/bevyengine/bevy/pull/10231/files#diff-4bf3ed03d4129aad9f5678ba19f9b14ee8e3e61d6f6365e82197b01c74468b10R472-R484. MeshPipeline::get_batch_data depends on data populated by build_lightmap_texture_arrays, so build_lightmap_texture_arrays needs to run first. If there's another way you'd prefer me to do this, let me know.

[pcwalton]

Closed as this is no longer blocking anything.