cssshadowvalue: Add a cache for blurred Pango layouts

cssshadowvalue: Add a cache for blurred Pango layouts

Drawing text with Pango is quite expensive, and drawing text and also
blurring it is *really* expensive. To prevent us from drawing a lot of
text and then blurring it a lot is *really* expensive.

We now cache the blurred pixels for the last layout and shadow we made,
which means we can repeatedly draw labels with a blurred text-shadow
extremely fast.

To detect whether the shadow is up-to-date, we track the serial of the
PangoLayout alongside the radius of the box shadow. We don't support
inset shadows nor spread on text-shadow, so we don't need to track
these.