We cache the last processed model index as well as the used
font. In some cases the cached information interfered with
new information and ended up in an ugly painting issue
where old information partially got repainted instead of
using the new. Clear cached information when starting a new
run to avoid this.

Change-Id: Ibb01e5e3bdd7b542325b5867d37c677d31a14aae
Reviewed-by: David Schulz <david.schulz@qt.io>