Add a debug function to save contents of distance field cache

Just copy the one from QSGSharedDistanceFieldCache with some
compile fixes to the superclass so we store the default cache
as well.

Change-Id: I1fcc390601eea58f8b7729c9cead418e4c94714c
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>