wined3d: Set the current location to SFLAG_INTEXTURE instead of SFLAG_INDRAWABLE in wined3d_surface_depth_blt().

These currently end up being the same thing, but that will change soon.

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.