By default, all widgets are double buffered so this patch removes two
calls to gtk_widget_set_double_buffered() that are only enabling it.

The only special case is under egl (virgl) at set_egl_enabled() as
disabling the double buffered seems to prevent glitches. The call to
gtk_widget_set_double_buffered() was kept but wrapped under
G_GNUC_BEGIN/END_IGNORE_DEPRECATIONS to avoid warnings.

Note however that this is only expected to work under X11.

Warnings fixed:
 | spice-widget.c: In function ‘spice_display_init’:
 | spice-widget.c:643:5: warning: ‘gtk_widget_set_double_buffered’ is
 | deprecated
 |
 |     gtk_widget_set_double_buffered(area, true);
 |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 |
 | spice-widget.c:661:5: warning: ‘gtk_widget_set_double_buffered’ is
 | deprecated
 |
 |     gtk_widget_set_double_buffered(area, true);
 |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 |
 | spice-widget.c: In function ‘set_egl_enabled’:
 | spice-widget.c:1290:9: warning: ‘gtk_widget_set_double_buffered’ is
 | deprecated
 |     gtk_widget_set_double_buffered(GTK_WIDGET(area), !enabled);
 |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>