Use enum wl_pointer_button_state instead of integer

Instead of using a uint32_t for state everywhere (except on the wire,
where that's still the call signature), use the new
wl_pointer_button_state enum, and explicit comparisons.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/src/compositor.h b/src/compositor.h
index 08dcd77..1ea2de5 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -478,7 +478,7 @@
 	      wl_fixed_t x, wl_fixed_t y);
 void
 notify_button(struct wl_seat *seat, uint32_t time, int32_t button,
-	      uint32_t state);
+	      enum wl_pointer_button_state state);
 void
 notify_axis(struct wl_seat *seat, uint32_t time, uint32_t axis,
 	    int32_t value);