Tweak overbright indicator icon to be more visible on white background

An outline is now present on all sides to ensure it's more visible
when the "new" color (on the right of a ColorPicker) is overbright.

editor/icons/OverbrightIndicator.svg

@@ -1 +1 @@
-<svg height="16" viewBox="0 0 16 16" width="16" xmlns="http://www.w3.org/2000/svg"><path d="m0 0v10l10-10z" fill="#fff"/><path d="m0 12 12-12h-2l-10 10z" fill="#000003"/></svg>
+<svg height="16" viewBox="0 0 16 16" width="16" xmlns="http://www.w3.org/2000/svg"><path d="m.5.5v10l10-10z" fill="#fff" stroke="#000"/><path d="m0 12 12-12h-1.714286l-10.285714 10.285714z" fill="#000003" stroke-width="2"/></svg>