elm/win: don't force opaque bg if alpha is set

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