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.

It should not be needed anymore, now the wined3d context is implicitly
validated on GL context activation. The check itself wasn't reliable
because of some threading issues and it also brought a small performance hit.