sal_uInt32 seems reasonable given that this is the number of rectanges
in an image.

And then convert all of the other BeginSetClipRegion methods to use
sal_uInt32 too.

Change-Id: I85a712ec823662ac30f3859051e2b974fb99775e
Reviewed-on: https://gerrit.libreoffice.org/68343
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>