libtxt: Text shadow

[cooler-king]

Add support for text shadowing as in CSS.

Or point me to it if it already exists; doesn't seem to be in TextStyle or TextDecoration.

[cooler-king]

So this won't be addressed any time soon?

[abarth]

Looks like this is currently prioritized as something we want in the intermediate term but not something we need immediately.

[dvdwasibi]

Customer Pink requires this feature.

[Hixie]

@dvdwasibi You can fake it for now by blurring one text widget at on offset, then putting another on top.