fixed video still "snap on sprite" for Firefox

HISTORY.md

@@ -80,6 +80,7 @@
 
 ### 2019-05-09
 * Blocks, Objects, Threads: tweaked new video-snap (still capture) feature 
+* Objects: fixed video still "snap on sprite" for Firefox
 
 ### 2019-05-08
 * Blocks, Objects, Threads: integrated video capture control into global settings prims in Sensing

snap.html

@@ -8,7 +8,7 @@
         <script type="text/javascript" src="src/widgets.js?version=2019-04-05"></script>
         <script type="text/javascript" src="src/blocks.js?version=2019-05-08"></script>
         <script type="text/javascript" src="src/threads.js?version=2019-05-09"></script>
-        <script type="text/javascript" src="src/objects.js?version=2019-05-09"></script>
+        <script type="text/javascript" src="src/objects.js?version=2019-05-09_2"></script>
         <script type="text/javascript" src="src/gui.js?version=2019-05-08"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-02-22"></script>
         <script type="text/javascript" src="src/lists.js?version=2019-04-27"></script>

src/objects.js

@@ -1978,9 +1978,9 @@ SpriteMorph.prototype.videoSnap = function() {
     );
     snap = newCanvas(new Point(cst.width, cst.height), true);
     ctx = snap.getContext('2d');
-    ctx.drawImage(stage.videoLayer(), -offset.x, -offset.y);
-    ctx.globalCompositeOperation = 'destination-atop';
     ctx.drawImage(cst, 0, 0);
+    ctx.globalCompositeOperation = 'source-in';
+    ctx.drawImage(stage.videoLayer(), -offset.x, -offset.y);
     return new Costume(snap, this.newCostumeName(localize('snap')));
 };