emit 'plotly_webglcontextlost' event on webgl context lost

- with event data containing the original event and
  the canvas 'key' to determine which canvas lost its context.

Author: etpinard

src/lib/prepare_regl.js

@@ -48,6 +48,15 @@ module.exports = function prepareRegl(gd, extensions) {
         } catch(e) {
             success = false;
         }
+
+        if(success) {
+            this.addEventListener('webglcontextlost', function(event) {
+                gd.emit('plotly_webglcontextlost', {
+                    event: event,
+                    layer: d.key
+                });
+            }, false);
+        }
     });
 
     if(!success) {