simple-egl: Set the opaque region if windowed and -o is passed

simple-egl: Set the opaque region if windowed and -o is passed

Since commit 6a615d262141de7cf094788203d9c044dfb9f08d [1], the opaque
region would be set only when running fullscreen. Having it set
properly for the windowed case is helpful to test the overlay path in
compositor-drm.

What this patch does is:

 - reverts the above commit;
 - remove the "if fullscreen make the window opaque" conditional, that
   should have been removed when -o was introduced and was actually the
   cause for the bug solved in [1];
 - sets the opaque region when running fullscreen, regardless of the -o
   switch.

[1] commit 6a615d262141de7cf094788203d9c044dfb9f08d
    Author: Scott Moreau <oreaus@gmail.com>
    Date:   Thu Aug 30 14:44:16 2012 -0600

        simple-egl: Only set alpha_size=0 when -o is passed.

v2: - Clarify in the commit message that this does not regress the bug
      solved in [1].
    - Use the correct sha1 for the reverted commit.