elm_panel: decide whether a scrollable panel is open or not only when it has a size

elm_panel: decide whether a scrollable panel is open or not only when it has a size

in _state_sync() function, scrollable content's position is used to tell
whether the panel is open or not. in case of a top panel, for example,
it is considered to be open when the content's vertical position equals to zero.
however, this logic has a defect in that the position is always zero
if the content size is zero - not resized properly yet. it caused unwanted results.
this patch added some codes to check
1. if the panel's width or height is bigger than zero
2. if the box which contains panel contents is visible (resizable)

@fix