Add a dummy NoneCharAtlas implementation

This adds the logic needed to switch between multiple char atlas
implementations, and an example second implementation to test it out
with.

This also offers an escape hatch if something goes wrong with the char
atlas implementation(s).

demo/index.html

@@ -53,6 +53,15 @@
           <p>
             <label>tabStopWidth <input type="number" id="option-tabstopwidth" value="8" /></label>
           </p>
+          <p>
+            <label>
+              experimentalCharAtlas
+              <select id="option-experimental-char-atlas">
+                <option value="static" selected>static</option>
+                <option value="none">none</option>
+              </select>
+            </label>
+          </p>
           <div>
           	<h3>Size</h3>
             <div>