Proper rounding for possibly negative numbers

src/ol/render/canvas/canvasimmediate.js

@@ -274,8 +274,8 @@ ol.render.canvas.Immediate.prototype.drawImages_ =
     var x = pixelCoordinates[i] - this.imageAnchorX_;
     var y = pixelCoordinates[i + 1] - this.imageAnchorY_;
     if (this.imageSnapToPixel_) {
-      x = (x + 0.5) | 0;
-      y = (y + 0.5) | 0;
+      x = Math.round(x);
+      y = Math.round(y);
     }
     if (rotation !== 0 || this.imageScale_ != 1) {
       var centerX = x + this.imageAnchorX_;

src/ol/render/canvas/canvasreplay.js

@@ -329,8 +329,8 @@ ol.render.canvas.Replay.prototype.replay_ = function(
           x = pixelCoordinates[d] - anchorX;
           y = pixelCoordinates[d + 1] - anchorY;
           if (snapToPixel) {
-            x = (x + 0.5) | 0;
-            y = (y + 0.5) | 0;
+            x = Math.round(x);
+            y = Math.round(y);
           }
           if (scale != 1 || rotation !== 0) {
             var centerX = x + anchorX;