fix drawing regl lines only cases on shared WebGL

Author: archmoj

src/traces/scattergl/plot.js

@@ -60,6 +60,14 @@ var exports = module.exports = function plot(gd, subplot, cdata) {
     linkTraces(gd, subplot, cdata);
 
     if(scene.dirty) {
+        if(
+            (scene.line2d || scene.error2d) &&
+            !(scene.scatter2d || scene.fill2d || scene.glText)
+        ) {
+            // Fixes shared WebGL context drawing lines only case
+            regl.clear({});
+        }
+
         // make sure scenes are created
         if(scene.error2d === true) {
             scene.error2d = createError(regl);

test/image/compare_pixels_test.js

@@ -113,18 +113,6 @@ for(var i = 0; i < allMockList.length; i++) {
         continue;
     }
 
-    // the following mocks than only have regl-line2d lines (no morkers),
-    // do not render at first draw when using virtual webgl. TODO: find a fix
-    if(virtualWebgl && [
-        'gl2d_connect_gaps',
-        'gl2d_line_select',
-        'gl2d_lines_almost_horizontal_vertical',
-        'gl2d_scattergl_simple_line_reversed_ranges',
-        'gl2d_subplots_anchor'
-    ].indexOf(mockName) !== -1) {
-        continue;
-    }
-
     var isMapbox = mockName.substr(0, 7) === 'mapbox_';
     var isOtherFlaky = [
         // list flaky mocks other than mapbox: