[api-minor] Deprecate the imageLayer functionality

This functionality has never been used anywhere in the PDF.js library/viewer itself, since it was added in 2013.
Furthermore this functionality is, and has always been, *completely untested* and also unmaintained.
Finally, there's (at least) one old issue about `appendImage` not returning the correct position; see issue 4182.

All-in-all, it seems that keeping very old, untested, unmaintained, and partially broken code around probably isn't what we want here.
(On the off-chance that any future a11y-work requires getting access to image-positions, it'd likely be much better to re-implement the necessary functionality from scratch and also make sure that it's properly tested from the beginning.)

src/display/canvas.js

@@ -13,6 +13,13 @@
  * limitations under the License.
  */
 
+import {
+  deprecated,
+  getCurrentTransform,
+  getCurrentTransformInverse,
+  getRGB,
+  PixelsPerInch,
+} from "./display_utils.js";
 import {
   FeatureTest,
   FONT_IDENTITY_MATRIX,
@@ -26,12 +33,6 @@ import {
   Util,
   warn,
 } from "../shared/util.js";
-import {
-  getCurrentTransform,
-  getCurrentTransformInverse,
-  getRGB,
-  PixelsPerInch,
-} from "./display_utils.js";
 import {
   getShadingPattern,
   PathType,
@@ -1184,6 +1185,9 @@ class CanvasGraphics {
     this.baseTransform = getCurrentTransform(this.ctx);
 
     if (this.imageLayer) {
+      deprecated(
+        "The `imageLayer` functionality will be removed in the future."
+      );
       this.imageLayer.beginLayout();
     }
   }