always set alpha for internal wins

due to the presence of color classes on internal windows, e must
ensure that users who set translucent colors for window base objects
get transparency as expected on those parts of the window. it's impossible
to determine in advance whether alpha is needed, so it's best to just
set alpha in all cases

fix T2050

src/bin/e_win.c

@@ -479,6 +479,7 @@ e_elm_win_add(Evas_Object *parent, const char *name, Elm_Win_Type type)
    e_util_env_set("ELM_ACCEL", "none");
    o = elm_win_add(parent, name, type);
    e_util_env_set("ELM_ACCEL", eng);
+   elm_win_alpha_set(o, 1);
    free(eng);
    return o;
 }