Fix #7798: Refactor scratch canvas usage.

Fixes extra canvas create calls.
Fixes unnecessary call of `new DOMCanvasFactory`.
Fixes undefined error of DOMCanvasFactory.
Fixes failures in some of the tests.
Fixes expected behaviour.
Remove unused vars.

src/display/font_loader.js

@@ -216,6 +216,7 @@ if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('MOZCENTRAL')) {
 
       var i, ii;
 
+      // The temporary canvas is used to determine if fonts are loaded.
       var canvas = document.createElement('canvas');
       canvas.width = 1;
       canvas.height = 1;