Update pixelRef genID when reusing pixels in RecyclingClippingPixelAllocator.

When this code was refactored to support the new RegionDecoder it appears that
we dropped a call to bump the genID.  Adding it back is functionally correct
and should fix the recycling issue.

bug: 26617759
Change-Id: I966d398ca983edb40040e01345799b3cc9957fe0