[Editor] Add a way to extract the outlines of a union of rectangles

The goal is to be able to get these outlines to fill the shape corresponding
to a text selection in order to highlight some text contents.
The outlines will be used either to show selected/hovered highlights.

web/draw_layer_builder.js

@@ -0,0 +1,58 @@
+/* Copyright 2022 Mozilla Foundation
+ *
+ * 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.
+ */
+
+/** @typedef {import("../src/display/draw_layer.js").DrawLayer} DrawLayer */
+
+import { DrawLayer } from "pdfjs-lib";
+
+/**
+ * @typedef {Object} DrawLayerBuilderOptions
+ * @property {DrawLayer} [drawLayer]
+ */
+
+class DrawLayerBuilder {
+  #drawLayer = null;
+
+  /**
+   * @param {DrawLayerBuilderOptions} options
+   */
+  constructor(options) {
+    this.pageIndex = options.pageIndex;
+  }
+
+  /**
+   * @param {string} intent (default value is 'display')
+   */
+  async render(intent = "display") {
+    if (intent !== "display" || this.#drawLayer || this._cancelled) {
+      return;
+    }
+    this.#drawLayer = new DrawLayer({
+      pageIndex: this.pageIndex,
+    });
+  }
+
+  cancel() {
+    this._cancelled = true;
+
+    if (!this.#drawLayer) {
+      return;
+    }
+    this.#drawLayer.destroy();
+    this.#drawLayer = null;
+  }
+}
+
+export { DrawLayerBuilder };