Invert `bevy_gltf` dependency with `bevy_pbr`

Previously, bevy_gltf depended on bevy_pbr. This meant scene definition was tightly coupled to rendering. This dependency has been inverted, to allow bevy_gltf to function without any of the rendering stack present.

bevy_gltf is also an optional dependency.

In 0.18, loading a material sub-asset would return a Handle<StandardMaterial>.

let handle: Handle<StandardMaterial> = asset_server.load("models/animated/Fox.glb#Material0");

In 0.19, loading a material sub-asset loads a GltfMaterial to accurately represent the data in the glTF file. To load the StandardMaterial, use the /std suffix when the bevy_pbr feature is turned on (the feature is on by default).

let handle: Handle<GltfMaterial> = asset_server.load("models/animated/Fox.glb#Material0");
let handle_std: Handle<StandardMaterial> = asset_server.load("models/animated/Fox.glb#Material0/std");

You can disable PBR rendering by initializing PbrPlugin as so:

PbrPlugin {
    gltf_enable_standard_materials: false,
    ..Default::default()
}

GltfExtensionHandler trait's methods have been updated:

• on_material passes in the material_asset : &GltfMaterial and material_label: &str
• on_spawn_mesh_and_material also passes in the material_label: &str

UvChannel has moved from bevy_pbr to bevy_mesh.