Use try/catch to detect undefined globals for canvas

common/lib/xmodule/xmodule/js/src/capa/schematic.js

@@ -3339,7 +3339,9 @@ schematic = (function() {
 	}
 
 	// add method to canvas to compute relative coords for event
-	if (HTMLCanvasElement) HTMLCanvasElement.prototype.relMouseCoords = function(event){
+	try {
+		if (HTMLCanvasElement)
+		 HTMLCanvasElement.prototype.relMouseCoords = function(event){
 		    // run up the DOM tree to figure out coords for top,left of canvas
 		    var totalOffsetX = 0;
 		    var totalOffsetY = 0;
@@ -3357,6 +3359,9 @@ schematic = (function() {
 		    this.page_x = event.pageX;
 		    this.page_y = event.pageY;
 		 }
+	}
+	catch (err) { // ignore
+	}
 
 	///////////////////////////////////////////////////////////////////////////////
 	//
@@ -4091,6 +4096,8 @@ schematic = (function() {
 
 	// add dashed lines!
 	// from http://davidowens.wordpress.com/2010/09/07/html-5-canvas-and-dashed-lines/
+	try {
+		if (CanvasRenderingContext2D)
 			CanvasRenderingContext2D.prototype.dashedLineTo = function(fromX, fromY, toX, toY, pattern) {
 			    // Our growth rate for our line can be one of the following:
 			    //   (+,+), (+,-), (-,+), (-,-)
@@ -4132,7 +4139,9 @@ schematic = (function() {
 				dash = !dash;
 			    }
 			};
-
+	}
+	catch (err) { //noop
+	}
 	// given a range of values, return a new range [vmin',vmax'] where the limits
 	// have been chosen "nicely".  Taken from matplotlib.ticker.LinearLocator
 	function view_limits(vmin,vmax) {