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'.

terminal.c

@@ -2181,7 +2181,7 @@ term_fractional_scaling(const struct terminal *term)
 bool
 term_preferred_buffer_scale(const struct terminal *term)
 {
-    return term->wl->has_wl_compositor_v6 && term->window->preferred_buffer_scale > 0;
+    return term->window->preferred_buffer_scale > 0;
 }
 
 bool