ui/gtk: set scanout mode in gd_egl/gd_gl_area_scanout_texture

ui/gtk: set scanout mode in gd_egl/gd_gl_area_scanout_texture

Fixing a regression (black screen) caused by a commit 92b58156e7
("ui/gtk: set scanout-mode right before scheduling draw").

The commit 92b58156e7 was made with an assumption that the scanout
mode needs to be set only if the guest scanout is a dmabuf but there
are cases (e.g. virtio-gpu-virgl) where the scanout is still processed
in a form of a texture but is not backed by dmabuf. So it is needed
to put back the line that sets scanout mode in gd_egl_scanout_texture
and gd_gl_area_scanout_texture.

Fixes: 92b58156e7 ("ui/gtk: set scanout-mode right before scheduling draw)
Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230725001131.24017-1-dongwon.kim@intel.com>