Avoid creating intermediate strings in Util.makeCssRgb().

On one example (the Wuppertal map) this avoids the creation of over
230,000 intermediate strings.
In the reftest analyzer, make it possible to toggle "Circle differences" with the D key

It's currently possible to step through the test results using the <kbd>N</kbd> and <kbd>P</kbd> keys, and you can also switch between test and reference images with the <kbd>T</kbd> key.
However if you want to highlight the differences, that can only be done by clicking. This has always annoyed me somewhat, so this patch adds support for toggling the differences view with the <kbd>D</kbd> key.
Merge pull request #5050 from Snuffleupagus/reftest-analyzer-differences-key

In the reftest analyzer, make it possible to toggle "Circle differences" with the D key
Merge pull request #5033 from nnethercote/makeCssRgb

Avoid creating intermediate strings in Util.makeCssRgb().
Added find match counter