We'll probably want to make this default to TRUE at some point, but at least
for the moment there still seem to be some performance advantages to rendering
onscreen in most cases.

wined3d: Don't get rid of the user memory unless we're actually called with NULL in surface_set_mem().

In particular, some applications will call surface_set_mem() with the same
pointer to notify us the contents have changed.

There's no need for it now that we open the package in transacted mode.