AGS: Fix seg fault in Dreams in Witch House

[antoniou79]

Reported here https://bugs.scummvm.org/ticket/14338

Please review. This line fixes the issue, and I don't see any obvious side-effects, but someone with better knowledge of the engine should approve or dismiss the fix.

[tag2015]

This segfault is fixed in the recent upstream commits that reworked the SpriteBatches, but since those cause some regressions (even with the upstream interpreter) with old games, I'm not too sure we should merge those already (or at all).
This fix seems fine to me.

[criezy]

I was a bit worried about the potential impact of this change. So I pushed a different fix instead in 4283861.

As I see it the main issue was that a double-free happened on a Bitmap object because a static_pointer_cast was used to copy the Bitmap from one std::shared_ptr<Bitmap> to another. That mean that two separate std::shared_ptr were pointing to the same Bitmap object using a different tracker. It seems to me that the proper way to fix is to use the std::shared_ptr::operator=() to do the copy, and this is the change I committed.