Update version numbers and changelog for v0.12.0 release.

`cargo-semver-checks` says that `tiny-skia-path` also requires a
semver-breaking release (and it looks like historically it's version has
been locked to that of `tiny-skia` anyway), so I've also updated it's
version number.

Signed-off-by: Nico Burns <nico@nicoburns.com>