[Plot] Fall back from WebGL on context loss

When a WebGL context is loss, fall back to displaying
plots using regular canvas 2d API. WTD-475.

platform/features/plot/src/MCTChart.js

@@ -155,6 +155,7 @@ define(
                     }
                 }
 
+                // Switch from WebGL to plain 2D if context is lost
                 function fallbackFromWebGL() {
                     element.html(TEMPLATE);
                     canvas = element.find("canvas")[0];
@@ -173,6 +174,11 @@ define(
                     return;
                 }
 
+                // WebGL is a bit of a special case; it may work, then fail
+                // later for various reasons, so we need to listen for this
+                // and fall back to plain canvas drawing when it occurs.
+                canvas.addEventListener("webglcontextlost", fallbackFromWebGL);
+
                 // Check for resize, on a timer
                 activeInterval = $interval(drawIfResized, 1000);