color range slider mask rect with Color.fill

- so that rgba color is set as:
  `fill: ${rgb}` + `fill-opacity: ${alpha}`
  for compatibility with eps/pdf exports

Author: etpinard

src/components/rangeslider/draw.js

@@ -406,10 +406,9 @@ function drawMasks(rangeSlider, gd, axisOpts, opts) {
         .classed(constants.maskMinClassName, true)
         .attr({ x: 0, y: 0 });
 
-    maskMin.attr({
-        height: opts._height,
-        fill: constants.maskColor
-    });
+    maskMin
+        .attr('height', opts._height)
+        .call(Color.fill, constants.maskColor);
 
     var maskMax = rangeSlider.selectAll('rect.' + constants.maskMaxClassName)
         .data([0]);
@@ -418,10 +417,9 @@ function drawMasks(rangeSlider, gd, axisOpts, opts) {
         .classed(constants.maskMaxClassName, true)
         .attr('y', 0);
 
-    maskMax.attr({
-        height: opts._height,
-        fill: constants.maskColor
-    });
+    maskMax
+        .attr('height', opts._height)
+        .call(Color.fill, constants.maskColor);
 }
 
 function drawSlideBox(rangeSlider, gd, axisOpts, opts) {