get the right _xlength for surface if x is 2D

There are a bunch of extra edge cases here if the x/y/z arrays
don't have one of the expected sizes/dimensionalities... but this
covers more than it used to anyhow, and gets the ribbons mock right.

Author: alexcjohnson

src/traces/surface/convert.js

@@ -200,7 +200,7 @@ proto.update = function(data) {
         yaxis = sceneLayout.yaxis,
         zaxis = sceneLayout.zaxis,
         scaleFactor = scene.dataScale,
-        xlen = data._xlength,
+        xlen = z[0].length,
         ylen = data._ylength,
         coords = [
             ndarray(new Float32Array(xlen * ylen), [xlen, ylen]),

src/traces/surface/defaults.js

@@ -29,12 +29,12 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
         return;
     }
 
-    traceOut._xlength = z[0].length;
-    traceOut._ylength = z.length;
-
-    coerce('x');
+    var x = coerce('x');
     coerce('y');
 
+    traceOut._xlength = (Array.isArray(x) && Array.isArray(x[0])) ? z.length : z[0].length;
+    traceOut._ylength = z.length;
+
     var handleCalendarDefaults = Registry.getComponentMethod('calendars', 'handleTraceDefaults');
     handleCalendarDefaults(traceIn, traceOut, ['x', 'y', 'z'], layout);