This sets the FreetypeFont pointer of the FreetypeFontInstances
to nullptr when clearing the cache.
And it changes the interface functions of SalLayoutGlyphs to some
variant different from std::vector.

I don't know if we should prefer the mutable or the const font
instance. With mutable at least one can invalidate the font
instance when checking the IsValid(), so we can get rid of our
referenced font instance.

Change-Id: I6070cfcb3c549dbad3383bd4ec2b05b30645b753
Reviewed-on: https://gerrit.libreoffice.org/62688
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>