Implement the new spotlight layout

src/grid/CallLayout.ts

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+/*
+Copyright 2024 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { BehaviorSubject, Observable } from "rxjs";
+
+import { MediaViewModel } from "../state/MediaViewModel";
+import { LayoutSystem } from "./Grid";
+import { Alignment } from "../room/InCallView";
+
+export interface Bounds {
+  width: number;
+  height: number;
+}
+
+export interface CallLayoutInputs {
+  /**
+   * The minimum bounds of the layout area.
+   */
+  minBounds: Observable<Bounds>;
+  /**
+   * The alignment of the floating tile, if any.
+   */
+  floatingAlignment: BehaviorSubject<Alignment>;
+}
+
+export interface CallLayoutOutputs<Model> {
+  /**
+   * The visually fixed (non-scrolling) layer of the layout.
+   */
+  fixed: LayoutSystem<Model, MediaViewModel[], HTMLDivElement>;
+  /**
+   * The layer of the layout that can overflow and be scrolled.
+   */
+  scrolling: LayoutSystem<Model, MediaViewModel, HTMLDivElement>;
+}
+
+/**
+ * A layout system for media tiles.
+ */
+export type CallLayout<Model> = (
+  inputs: CallLayoutInputs,
+) => CallLayoutOutputs<Model>;