elm/win: don't force opaque bg if alpha is set
Summary:
alpha prevents an opaque bg, so ensure that this is being correctly
detected
worth noting here is that the eo api for win calls through _elm_win_standard_init()
during finalize which ensures that need_bg_solid is set; the legacy api
does not call this during "normal" construction
fix T7129
Depends on D6568
Reviewers: devilhorns
Subscribers: cedric, #committers
Tags: #efl
Maniphest Tasks: T7129
Differential Revision: https://phab.enlightenment.org/D6569