interface_scrollable: Improve gravity_set api to support pan changing
When you set gravity 1 on scroller, scroller sticks to the bottom
even content is changed.
however, scroller don't work like above, if size of pan is changed.
this commit uses pan_pos_max rather than w/h of content_info
because pan_pos_max is related with both content_size and pan size.
gravity_set will work properly even if both size of content and pan are
- Select 'scroll3' in the elementary_test
- Append enough items so that scroll bar appears (about 30 items)
- Go to the bottom and Set gravity 1.0
- Check that scroller sticks to the bottom once you append another item (it works)
- Check that scroller sticks to to bottom once you resize window(pan) (it doesn't work without this patch)
Reviewers: eagleeye, jpeg, cedric, woohyun, z-wony, herdsman
Differential Revision: https://phab.enlightenment.org/D4665