Function prepare_morph_descriptors 

pub fn prepare_morph_descriptors(
    morph_indices: ResMut<'_, MorphIndices>,
    morph_uniforms: ResMut<'_, MorphUniforms>,
    render_mesh_instances: Res<'_, RenderMeshInstances>,
    meshes: Res<'_, RenderAssets<RenderMesh>>,
    mesh_allocator: Res<'_, MeshAllocator>,
)

A system that writes GpuMorphDescriptor values to the MorphUniforms for each mesh instance with morph targets.

As morph descriptors are only used when the platform supports storage buffers, if the platform doesn’t support storage buffers, this system does nothing.