[cairo] scaled_font lifecycle issue

[Lawrence D'Oliveiro]

On Sat, 9 Apr 2016 19:30:01 +0930, Adrian Johnson wrote:

> Maybe we need a font_face API function to make cairo drop a font from
> the cache once the application drops all references to the font_face
> and scaled fonts?

A more general invalidate-cache-for-specified-font-face function
would be very useful. It would help with use of multiple-master-type
font technologies. I hit this situation when implementing my wrapper
for the Hershey fonts <https://github.com/ldo/hersheypy> for Cairo: to
use different line widths, I had to load multiple copies of the same
font to get around Cairo’s caching.