rm 'blend' special setBackground value

- and make 'opaque' use Color.combine instead of hard-setting
  _paperdiv.

Author: etpinard

src/plot_api/plot_api.js

@@ -400,11 +400,6 @@ function setBackground(gd, bgColor) {
 }
 
 function opaqueSetBackground(gd, bgColor) {
-    gd._fullLayout._paperdiv.style('background', 'white');
-    setBackground(gd, bgColor);
-}
-
-function blendSetBackground(gd, bgColor) {
     var blend = Color.combine(bgColor, 'white');
     setBackground(gd, blend);
 }
@@ -421,12 +416,8 @@ function setPlotContext(gd, config) {
             key = keys[i];
             if(key === 'editable' || key === 'edits') continue;
             if(key in context) {
-                if(key === 'setBackground') {
-                    if(config[key] === 'opaque') {
-                        context[key] = opaqueSetBackground;
-                    } else if(config[key] === 'blend') {
-                        context[key] = blendSetBackground;
-                    }
+                if(key === 'setBackground' && config[key] === 'opaque') {
+                    context[key] = opaqueSetBackground;
                 } else {
                     context[key] = config[key];
                 }

src/plot_api/plot_config.js

@@ -106,9 +106,9 @@ module.exports = {
     // increase the pixel ratio for Gl plot images
     plotGlPixelRatio: 2,
 
-    // function to add the background color to a different container
-    // or 'opaque' to ensure there's white behind it,
-    // or 'blend' to blend bg color with white,
+    // background setting function
+    // 'transparent' sets the background `layout.paper_color`
+    // 'opaque' blends bg color with white ensuring an opaque background
     // or any other custom function of gd
     setBackground: 'transparent',
 

src/plot_api/to_image.js

@@ -47,7 +47,7 @@ var attrs = {
             'Sets the image background mode.',
             'By default, the image background is determined by `layout.paper_bgcolor`,',
             'the *transparent* mode.',
-            'One might consider setting `setBackground` to *opaque* or *blend*',
+            'One might consider setting `setBackground` to *opaque*',
             'when exporting a *jpeg* image as JPEGs do not support opacity.'
         ].join(' ')
     },