Improve new API

[Nicolas-Ferre]

Feature
Improve the new API:

• Use App everywhere instead of Context for more flexibility
• Avoid splitting data between local node and glob, and instead put everything on glob side
• Ensure valid state at any time by applying full glob update each time a glob field is modified

Tasks

• Replace Context by App #315
• Add FromApp #316
• Require FromApp to be implemented for all glob types #317
• Refactor globs #318
• Allow mutable borrowing of multiple root nodes #320
• Replace nodes by states #321
• Support Instant creation in FromApp derive macro #322
• Allow glob mutation with Globals #323
• Add updater traits and derive macros #324
• Change physics API #325
• Change resource API #326
•  Change material api #328
• Simplify updaters
  • Don't generate for_* methods (so updater fields can be Option instead of Update)
  • Generation *_option methods to simplify updaters calling other updaters
  • Generate updater fields with updater(inner_type) only to support custom methods
  • Rename attributes
    • Generate field only -> updater(field)
    • Generate methods + fields -> updater(methods)
• Add Init trait to initalize automatically after FromApp::from_app() call
• Make ... a glob a glob and implements GlobUpdater
• Optimize state index access in App (see Snippet 1)
  • Actually, current version is convenient to have a way to control state update ordering.
• Avoid storing the GPU objects in an Arc, and use App::take instead
• Perform last global cleanup/refactoring if necessary, including:
  • Remove unnecessary app.update() in tests
  • Put most objects outside Root in tests to simplify borrowing
  • Remove FontManager and instead update all Text2D when the font is updated.
  • Make sure all structfield doc references are still valid.
• Allow deletion of states (and provide State::delete() callback, for example to delete recursively)
  • Possible issue: delete state of type T, and then run StateHandle::<T>::get
  • Possible issue: what happens to Globs when Globals state is deleted?
• Either replace GlobRef by Glob, or disallow mutation from GlobRef.
• Implement From/Into<[f32; N]> for Vec2, Vec3, ...
• Add ModelMapper to derivate a set of models dynamically (for picking, shadow maps, ...)
• Get access to shader + textures + raw material data + material type ID from MaterialGlob
• Add is_main_rendering field in Target
• Ensure that when TextureAnimation.part field is updated, the current frame is reset

Snippet 1:

fn root_index_or_create<T>(&mut self) -> usize
where
    T: RootNode,
{
    let type_id = TypeId::of::<T>();
    let index = *self
        .root_indexes
        .entry(type_id)
        .or_insert_with(|| self.roots.len());
    if index == self.roots.len() {
        debug!("Create root node `{}`...", any::type_name::<T>());
        // reserve slot in case other root node is creating during T::from_app_with
        let root = T::from_app_with(self, T::init);
        self.roots.push(RootNodeData::new::<T>(root));
        debug!("Root node `{}` created", any::type_name::<T>());
    }
    index
}