Possibly advances an animation to a new frame.
Chooses the frame based on the start time passed to get_iter .
current_time would normally come from TimeVal, and must
be greater than or equal to the time passed to get_iter,
and must increase or remain unchanged each time
get_pixbuf is called. That is, you can't go backward in time; animations only play forward.
As a shortcut, pass `NULL` for the current time and TimeVal will be
invoked on your behalf. So you only need to explicitly pass
current_time if you're doing something odd like playing the animation
at double speed.
If this function returns `FALSE`, there's no need to update the animation display, assuming the display had been rendered prior to advancing; if `TRUE`, you need to call get_pixbuf and update the display with the new pixbuf.
`TRUE` if the image may need updating