compositor: Add a switch_mode hook and a wrapper into the output.
This allow each output backend to switch the display mode. It is
useful to some applications like fullscreen game, xbmc.
diff --git a/src/compositor-x11.c b/src/compositor-x11.c
index 4361a01..87ab5dd 100644
--- a/src/compositor-x11.c
+++ b/src/compositor-x11.c
@@ -472,6 +472,7 @@
output->base.read_pixels = x11_output_read_pixels;
output->base.set_backlight = NULL;
output->base.set_dpms = NULL;
+ output->base.switch_mode = NULL;
wl_list_insert(c->base.output_list.prev, &output->base.link);