Replace contexts with resources

- Implements a new macro to generate code for a new structure: TypeMap
- TypeMaps are wrappers for HashMaps that use TypeIDs as keys.
- Refactor the entire codebase to use the new resource structures.
- This commit is the first step towards getting rid of "god context objects everywhere".

glium_platform/src/render/debug_ui.rs

@@ -41,7 +41,7 @@ impl RenderingSystem for ImguiBridge {
         self.rendered_this_frame = true;
 
         let ui = self.imgui.frame();
-        ctx.debug_ui_buffer.borrow().write_buffer(&ui);
+        ctx.debug_ui_buffer.write_buffer(&ui);
 
         {
             let window = ctx.window.lock().unwrap();