fixup download image when zsmooth is set to fast

Author: archmoj

src/traces/image/plot.js

@@ -26,7 +26,9 @@ module.exports = function plot(gd, plotinfo, cdimage, imageLayer) {
         var plotGroup = d3.select(this);
         var cd0 = cd[0];
         var trace = cd0.trace;
-        var realImage = supportsPixelatedImage && !trace._hasZ && trace._hasSource && xa.type === 'linear' && ya.type === 'linear';
+        var realImage = trace.zsmooth === 'fast' || (
+            supportsPixelatedImage && !trace._hasZ && trace._hasSource && xa.type === 'linear' && ya.type === 'linear'
+        );
         trace._realImage = realImage;
 
         var z = cd0.z;