Draw bars using fillRect instead of paths.

This is up to 2x faster and appears to work around issues in Chrome's
canvas implementation that sometimes result in bars not being filled.
Resolves #915.

jquery.flot.js

@@ -2544,13 +2544,8 @@ Licensed under the MIT license.
 
             // fill the bar
             if (fillStyleCallback) {
-                c.beginPath();
-                c.moveTo(left, bottom);
-                c.lineTo(left, top);
-                c.lineTo(right, top);
-                c.lineTo(right, bottom);
                 c.fillStyle = fillStyleCallback(bottom, top);
-                c.fill();
+                c.fillRect(left, top, right - left, bottom - top)
             }
 
             // draw outline