Engine: Initialize blend to 0 when inserting sprites
This fixes a bug where stale blend values could be transferred over from a previously loaded map.
To reproduce the bug:
Simply create a new board in Mapster32, place a single tile 0 sprite, and give it blend 129 (for example).
Then start a new board, create a sector and place a sprite in it. Check its blend value -- it will still be set to 129, despite this being a completely fresh, non-clipboard inserted sprite.