Update the definition of ReferenceImage.

- Add support for ImageData
- Breaking: remove support for HTMLCanvasElement

docs/api/reference-image.md

 
 ### image
 
-`referenceImage.image: HTMLImageElement | HTMLCanvasElement | ImageBitmap, read-only`
+`referenceImage.image: HTMLImageElement | ImageBitmap | ImageData, read-only`
 
-Image template with pixel data.
+Image template with pixel data.
+
+*Note:* `ImageData` is acceptable since version 0.4.0.

src/trackers/image-tracker/reference-image-database.ts

             throw new IllegalOperationError(`Can't add reference image "${name}" to the database: the capacity of ${this.capacity} images has been exceeded.`);
 
         // check if the image is valid
-        if(!(referenceImage.image instanceof HTMLImageElement) && !(referenceImage.image instanceof HTMLCanvasElement) && !(referenceImage.image instanceof ImageBitmap))
+        if(
+            !(referenceImage.image instanceof HTMLImageElement) &&
+            !(referenceImage.image instanceof ImageBitmap) &&
+            !(referenceImage.image instanceof ImageData)
+        )
             throw new IllegalArgumentError(`Can't add reference image "${name}" to the database: invalid image`);
 
         // check for duplicate names

src/trackers/image-tracker/reference-image.ts

 
 import { SpeedyMedia } from 'speedy-vision/types/core/speedy-media';
 
-type ReferenceImageType = HTMLImageElement | HTMLCanvasElement | ImageBitmap;
+type ReferenceImageType = HTMLImageElement | ImageBitmap | ImageData;
 
 
 
 export class ReferenceImageWithMedia implements ReferenceImage
 {
     /** The decorated reference image */
-    private _referenceImage: ReferenceImage;
+    private readonly _referenceImage: ReferenceImage;
 
     /** A SpeedyMedia corresponding to the reference image */
     private readonly _media: SpeedyMedia;
     }
 
     /**
-     * The image data
+     * Image data
      */
     get image(): ReferenceImageType
     {