Check fonts against two reference fonts

In rare cases, the previous font check returned false negatives when
checking against monospace only. By checking against another font, this
can be avoided.

src/ol/render/canvas.js

@@ -125,22 +125,26 @@ _ol_render_canvas_.checkFont = (function() {
   var retries = 60;
   var checked = _ol_render_canvas_.checkedFonts_;
   var labelCache = _ol_render_canvas_.labelCache;
-  var font = '32px monospace';
+  var size = '32px ';
+  var referenceFonts = ['monospace', 'serif'];
+  var len = referenceFonts.length;
   var text = 'wmytzilWMYTZIL@#/&?$%10';
   var interval, referenceWidth;
 
-  function isAvailable(fontFamily) {
+  function isAvailable(font) {
     var context = _ol_render_canvas_.getMeasureContext();
-    context.font = font;
-    referenceWidth = context.measureText(text).width;
     var available = true;
-    if (fontFamily != 'monospace') {
+    for (var i = 0; i < len; ++i) {
-      context.font = '32px ' + fontFamily + ',monospace';
+      var referenceFont = referenceFonts[i];
-      var width = context.measureText(text).width;
+      context.font = size + referenceFont;
-      // If width and referenceWidth are the same, then the 'monospace'
+      referenceWidth = context.measureText(text).width;
-      // fallback was used instead of the font we wanted, so the font is not
+      if (font != referenceFont) {
-      // available.
+        context.font = size + font + ',' + referenceFont;
-      available = width != referenceWidth;
+        var width = context.measureText(text).width;
+        // If width and referenceWidth are the same, then the fallback was used
+        // instead of the font we wanted, so the font is not available.
+        available = available && width != referenceWidth;
+      }
     }
     return available;
   }