Like ca381b61, this was introduced by
f1ca14cb.

Don't overwrite the entire buffer if only part of it is not current. The data
we're copying may not be completely current either. This is for the case where
the inner rect is not current and the outer edge is. The reverse case where
the inner rect is current and we need to mask already works correctly.

msvcrt: Ensure that old buffer contents are copied when allocating a growable pf_output buffer for the first time.