sprite: Always union with dirty_rect from Image
sprite: Always union with dirty_rect from Image

Whenever an Image is not dirty, the dirty rectangle from the Image spans
the dimensions of the whole Image, so this should allow us now to
optimize Image to skip transparent regions from updating.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

aszlig/hase