Implement fast zsmooth option for image trace

src/traces/image/attributes.js

@@ -51,6 +51,17 @@ module.exports = extendFlat({
             'otherwise it defaults to `rgb`.'
         ].join(' ')
     },
+    interpolate: {
+        valType: 'enumerated',
+        values: ['nearest', 'linear'],
+        role: 'info',
+        editType: 'plot',
+        description: [
+            'The type of interpolation for rendering the image.',
+            'Default `nearest` (nearest neighbor interpolation).',
+            'Can also be set to `linear` (bilinear interpolation).'
+        ].join(' ')
+    },
     zmin: {
         valType: 'info_array',
         items: [

src/traces/image/defaults.js

@@ -47,6 +47,7 @@ module.exports = function supplyDefaults(traceIn, traceOut) {
         traceOut.zmax = cm.zmaxDflt;
     }
 
+    coerce('interpolate', 'nearest');
     coerce('text');
     coerce('hovertext');
     coerce('hovertemplate');

src/traces/image/plot.js

@@ -139,7 +139,10 @@ module.exports = function plot(gd, plotinfo, cdimage, imageLayer) {
         // Pixelated image rendering
         // http://phrogz.net/tmp/canvas_image_zoom.html
         // https://developer.mozilla.org/en-US/docs/Web/CSS/image-rendering
-        var style = 'image-rendering: optimizeSpeed; image-rendering: -moz-crisp-edges; image-rendering: -o-crisp-edges; image-rendering: -webkit-optimize-contrast; image-rendering: optimize-contrast; image-rendering: crisp-edges; image-rendering: pixelated;';
+        var style = '';
+        if(trace.interpolate === 'nearest') {
+            style += 'image-rendering: optimizeSpeed; image-rendering: -moz-crisp-edges; image-rendering: -o-crisp-edges; image-rendering: -webkit-optimize-contrast; image-rendering: optimize-contrast; image-rendering: crisp-edges; image-rendering: pixelated;';
+        }
         if(fastImage) {
             var xRange = Lib.simpleMap(xa.range, xa.r2l);
             var yRange = Lib.simpleMap(ya.range, ya.r2l);