Sprite cache: Allow caching only required subset of sprite zoom levels

Enable for blitters based on 32bpp_optimized or SSE

src/blitter/null.cpp

@@ -24,6 +24,8 @@ Sprite *Blitter_Null::Encode(const SpriteLoader::Sprite *sprite, AllocatorProc *
 	dest_sprite->width  = sprite->width;
 	dest_sprite->x_offs = sprite->x_offs;
 	dest_sprite->y_offs = sprite->y_offs;
+	dest_sprite->next = nullptr;
+	dest_sprite->missing_zoom_levels = 0;
 
 	return dest_sprite;
 }