Support the maxCanvasPixels option in the thumbnails code

This addresses an inconsistency in the viewer, since the thumbnails don't respect the `maxCanvasPixels` option.
Note that, as far as I know, this has not lead to any bugs since the thumbnails render with a fixed (and small) width, however it really cannot hurt to address this (especially after the introduction of the `maxCanvasDim` option).

To support this a new `OutputScale`-method was added, to avoid having to duplicate code in multiple files.

src/display/display_utils.js

@@ -640,9 +640,39 @@ class OutputScale {
     return this.sx !== 1 || this.sy !== 1;
   }
 
+  /**
+   * @type {boolean} Returns `true` when scaling is symmetric,
+   *   `false` otherwise.
+   */
   get symmetric() {
     return this.sx === this.sy;
   }
+
+  /**
+   * @returns {boolean} Returns `true` if scaling was limited,
+   *   `false` otherwise.
+   */
+  limitCanvas(width, height, maxPixels, maxDim) {
+    let maxAreaScale = Infinity,
+      maxWidthScale = Infinity,
+      maxHeightScale = Infinity;
+
+    if (maxPixels > 0) {
+      maxAreaScale = Math.sqrt(maxPixels / (width * height));
+    }
+    if (maxDim !== -1) {
+      maxWidthScale = maxDim / width;
+      maxHeightScale = maxDim / height;
+    }
+    const maxScale = Math.min(maxAreaScale, maxWidthScale, maxHeightScale);
+
+    if (this.sx > maxScale || this.sy > maxScale) {
+      this.sx = maxScale;
+      this.sy = maxScale;
+      return true;
+    }
+    return false;
+  }
 }
 
 // See https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Image_types