Update webgl-demo.js

Calling gl.clear(...) explicitly.  Fixed running sample1 locally.

Author: dustin-biser

tutorial/sample1/webgl-demo.js

@@ -19,6 +19,7 @@ function start() {
     gl.clearDepth(1.0);                 // Clear everything
     gl.enable(gl.DEPTH_TEST);           // Enable depth testing
     gl.depthFunc(gl.LEQUAL);            // Near things obscure far things
+    gl.clear(gl.COLOR_BUFFER_BIT);      // Clear the color buffer with specified clear color
   }
 }
 
@@ -42,4 +43,4 @@ function initWebGL() {
   if (!gl) {
     alert("Unable to initialize WebGL. Your browser may not support it.");
   }
-}
+}