efl_ui_win: enable bg color set although alpha is set

efl_ui_win: enable bg color set although alpha is set

Summary:
This patch sets background transparent if window supports alpha.
If alpha window does not emit signal to show background rectangle,
then the background color set by _efl_ui_win_part_color_set cannot
be applied because the background rectangle remains hidden.

This patch fixes the side effect of D6569.

Test Plan: Run Efl.Ui.Win (Bg part) in elementary_test.

Reviewers: zmike, devilhorns, woohyun

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7302