Add option to rescale sprites

[cBournhonesque]

The sprites I use are of a given size, but in my game I like to use transform.scale to set the exact size of an entity, not to just scale it up or down.

Hence, I usually manually scale all sprites to Vec2(1.0, 1.0), so that the transform.scale is the final size of my object.

I think it would be useful to let users have this option. Another way they could do it is to load the particule image directly, compute it's size, and change the scale themselves accordingly, but it's somewhat of a hassle compared to just setting the custom_size to (1.0, 1.0)