Rename source types for consistency

src/core/session.ts

         // prefer video sources
         for(let i = 0; i < sources.length; i++) {
             const source = sources[i];
-            if(source._is('video'))
+            if(source._is('video-source'))
                 return source;
         }
         for(let i = 0; i < sources.length; i++) {
             const source = sources[i];
-            if(source._is('canvas'))
+            if(source._is('canvas-source'))
                 return source;
         }
 

src/sources/canvas-source.ts

      */
     _is<T extends keyof SourceType>(type: T): this is SourceType[T]
     {
-        return type === 'canvas';
+        return type === 'canvas-source';
     }
 
     /**

src/sources/source.ts

  * @internal
  */
 export type SourceType = {
-    'video': VideoSource,
-    'canvas': CanvasSource,
+    'video-source': VideoSource,
+    'canvas-source': CanvasSource,
     'pointer-source': PointerSource
 };

src/sources/video-source.ts

      */
     _is<T extends keyof SourceType>(type: T): this is SourceType[T]
     {
-        return type === 'video';
+        return type === 'video-source';
     }
 
     /**

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

         // XXX also let the user specify a source manually?
         for(const source of session.sources) {
             // prefer video sources
-            if(source._is('video')) {
+            if(source._is('video-source')) {
                 this._source = source;
                 break;
             }
-            else if(source._is('canvas'))
+            else if(source._is('canvas-source'))
                 this._source = source;
         }
         if(this._source === null)