set active region before calculating bounds

Actually `Sheet` was designed to have the whole image as active region by default, and if user overrides it with configuration of some function (see `Animator` above), then the image is treated as a sprite-cells and it may change through time (I have examples for this usage, but they are outdated :( ). Seems this logic fails somehow. May be we even don't need the `_active_region` variable, but always recalculate it for every call?