Mutable mesh accessors: indices_mut and attribute_mut

[msklywenn]

Hello,

In current bevy render, the only way to set attributes on a mesh is to set all values which involves a lot of cloning. For faster updating of dynamic meshes, I propose in this pull request to add two new accessors to Mesh so that it is possible to edit its values directly.

In this example, the iterator is copied directly to the mesh

let pos = sub.attribute_mut(Mesh::ATTRIBUTE_POSITION).unwrap();
if let VertexAttributeValues::Float3(ref mut pos) = pos {
    pos.clear();
    pos.extend(triangles.vertices.iter().map(|v| [v.pos.x, v.pos.y, 0.0f32]));
}

avoiding a collect() needed by the current approach

sub.set_attribute(Mesh::ATTRIBUTE_POSITION, VertexAttributeValues::Float3(triangles.vertices.iter().map(|v| [v.pos.x, v.pos.y, 0.0f32]).collect()));

[cart]

Looks good to me. Thanks!