clear regl colors once per drawData pass

- as opposed to once per basePlotModule pass,
  that way scattergl and splom can coexist
- this introduces an additional 'clear' call for parcoords
  trace. No side-effect were detected. But we should double-check
- add first cut splom + cartesian mock

Author: etpinard

src/traces/splom/base_plot.js

@@ -168,8 +168,6 @@ function showZeroLine(ax) {
 }
 
 function clean(newFullData, newFullLayout, oldFullData, oldFullLayout, oldCalcdata) {
-    oldCalcdata = oldCalcdata || [];
-
     var oldModules = oldFullLayout._modules || [];
     var newModules = newFullLayout._modules || [];
 

test/image/baselines/splom_with-cartesian.png

@@ -0,0 +1,38 @@
+{
+  "data": [{
+    "type": "splom",
+    "name": "splom",
+    "dimensions": [{
+      "values": [1, 2, 3],
+      "label": "A"
+    }, {
+      "values": [2, 5, 6],
+      "label": "B"
+    }]
+  }, {
+    "name": "scatter",
+    "y": [1, 2, 1]
+  }, {
+    "type": "box",
+    "name": "box",
+    "x0": 0,
+    "y": [1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 6],
+    "yaxis": "y2"
+  }, {
+    "type": "scattergl",
+    "name": "scattergl",
+    "y": [1, 2, 1],
+    "xaxis": "x2"
+  }],
+  "layout": {
+    "grid": {
+      "roworder": "bottom to top"
+    },
+    "legend": {
+      "x": 0,
+      "y": 1.1,
+      "yanchor": "bottom",
+      "orientation": "h"
+    }
+  }
+}