gamnit :: Sprite :: animation_start
sprite_set.timeAt -1.0 if animation started before being assigned a sprite_set.
# Animation start time, relative to `sprite_set.time`
#
# At -1.0 if animation started before being assigned a `sprite_set`.
private var animation_start = 0.0
lib/gamnit/flat/flat_core.nit:112,2--115,34