This was a regression in 3.1 and later from the new inspector, where
PROPERTY_HINT_SPRITE_FRAME was not fully re-implemented. It's meant to
be a normal PROPERTY_HINT_RANGE which also automatically increments its
value when keyed in the animation player.
To avoid code duplication, I made the frames properties use the actual
PROPERTY_HINT_RANGE and introduced a PROPERTY_USAGE_KEYING_INCREMENTS
usage flag instead.
336846e547