wayland: remove has_wl_compositor_v6

We deviate slightly from the specification, in that we don't assume a
preferred buffer scale of 1. Instead, we "guess" the scale *until we
receive a surface_preferred_buffer_scale event.

Because of this, we don't need the has_wl_compositor_v6 member, as
it's enough to check if we have a non-zero 'preferred buffer scale'.

wayland.h

@@ -430,8 +430,6 @@ struct wayland {
     struct wl_subcompositor *sub_compositor;
     struct wl_shm *shm;
 
-    bool has_wl_compositor_v6;
-
     struct zxdg_output_manager_v1 *xdg_output_manager;
 
     struct xdg_wm_base *shell;