User provided mesh binding / per-instance mesh data about bevy

[tychedelia]

@pcwalton Tagging you because you suggested this as a solution, and it's a great idea that would really help advance using bevy for art :). Also, in reviewing the feasibility of implementing this, the most gnarly part passes through the preprocessing compute shader bits which you wrote.

My approach is basically going to be this:

1. Add a new UserMeshInstancePlugin<T> that the user can add once per application, where T ensures proper alignment, etc.
2. In order not to pollute all the rendering code with a new type parameter, this is going to be type erased as RawBufferVec<u32>.
3. Pass this into the compute shader etc as an optional buffer that enables a shaderdef.
4. In the compute shader, bind input, output, and a push constant (or whatever) that indicates the size of the data.
5. After culling, write input to output at the same time as the mesh data.
6. In the user's vertex shader, they are thus able to do @group(0) @binding(6) var<storage> my_data: array<MyData>; and use the instance index to look up their per-mesh data.

Does this general approach make sense? Do you have any other more general advice for how to approach this?

[tychedelia]

One of the problems here is that we have a lot of nice wrappers around things like gpu buffers, creating an "erased" variant for each one is a bit unfortunate. If we permitted the pollution of lots of the rendering code with a user supplied generic type, it would significantly help the implementation, but I'm not sure how to implement the default case where there is no user supplied mesh instance data without specialization (which would still make the code much uglier).