Support stroked text

[ZevEisenberg]

I don't know whether there is a big demand for these (signs point to "no"), but I wanted to log it here in case someone wants it. Specifically, we should use an enum or something to abstract over this nonsense: https://developer.apple.com/library/content/qa/qa1531/_index.html.

[ZevEisenberg]

In case that URL goes dark:

Q: How can I draw a string that specifies both a fill color and a stroke width?
A: Supply a negative value for NSStrokeWidthAttributeName when you wish to draw a string that is both filled and stroked.

This is because the sign of the value for NSStrokeWidthAttributeName is interpreted as a mode; it indicates whether the attributed string is to be filled, stroked, or both. Specifically, a zero value displays a fill only, while a positive value displays a stroke only. A negative value allows displaying both a fill and stroke.