Clear the alphaCache when starting new painting

The alpha cache needs to be cleared when starting a new paint otherwise
it will assume that some elements already exist in the PDF when they
don't.

Change-Id: I97451c53c0c99b80a25a057735cef7b3bb830a7a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>