Release possible reference to source surface.
For performance reasons the last surface that has been used for Blit() and others stays
attached to the state of the destination surface to save the overhead of reprogramming
the same values each time.
That leads to the last source being still around regardless of it being released
via its own interface. The worst case is generation of thumbnails using StretchBlit()
from a huge surface to a small one. The small thumbnail surface keeps the big one alive,
because no other blitting will be done to the small surface afterwards.
To solve this, here's the method applications should use in such a case.
