Use multi_draw_indirect_count where available, in preparation for two-phase occlusion culling.

[pcwalton]

This commit allows Bevy to use multi_draw_indirect_count for drawing meshes. The multi_draw_indirect_count feature works just like multi_draw_indirect, but it takes the number of indirect parameters from a GPU buffer rather than specifying it on the CPU.

Currently, the CPU constructs the list of indirect draw parameters with the instance count for each batch set to zero, uploads the resulting buffer to the GPU, and dispatches a compute shader that bumps the instance count for each mesh that survives culling. Unfortunately, this is inefficient when we support multi_draw_indirect_count. Draw commands corresponding to meshes for which all instances were culled will remain present in the list when calling
multi_draw_indirect_count, causing overhead. Proper use of multi_draw_indirect_count requires eliminating these empty draw commands.

To address this inefficiency, this PR makes Bevy fully construct the indirect draw commands on the GPU instead of on the CPU. Instead of writing instance counts to the draw command buffer, the mesh preprocessing shader now writes them to a separate indirect metadata buffer. A second compute dispatch known as the build indirect parameters shader runs after mesh preprocessing and converts the indirect draw metadata into actual indirect draw commands for the GPU. The build indirect parameters shader operates on a batch at a time, rather than an instance at a time, and as such each thread writes only 0 or 1 indirect draw parameters, simplifying the current logic in mesh_preprocessing, which currently has to have special cases for the first mesh in each batch. The build indirect parameters shader emits draw commands in a tightly packed manner, enabling maximally efficient use of multi_draw_indirect_count.

Along the way, this patch switches mesh preprocessing to dispatch one compute invocation per render phase per view, instead of dispatching one compute invocation per view. This is preparation for two-phase occlusion culling, in which we will have two mesh preprocessing stages. In that scenario, the first mesh preprocessing stage must only process opaque and alpha tested objects, so the work items must be separated into those that are opaque or alpha tested and those that aren't. Thus this PR splits out the work items into a separate buffer for each phase. As this patch rewrites so much of the mesh preprocessing infrastructure, it was simpler to just fold the change into this patch instead of deferring it to the forthcoming occlusion culling PR.

Finally, this patch changes mesh preprocessing so that it runs separately for indexed and non-indexed meshes. This is because draw commands for indexed and non-indexed meshes have different sizes and layouts. The existing code is actually broken for non-indexed meshes, as it attempts to overlay the indirect parameters for non-indexed meshes on top of those for indexed meshes. Consequently, right now the parameters will be read incorrectly when multiple non-indexed meshes are multi-drawn together. This is a bug fix and, as with the change to dispatch phases separately noted above, was easiest to include in this patch as opposed to separately.