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/clients/terminal.c b/clients/terminal.c
index 27ab300..a0c6942 100644
--- a/clients/terminal.c
+++ b/clients/terminal.c
@@ -2222,13 +2222,14 @@
 static void
 button_handler(struct widget *widget,
 	       struct input *input, uint32_t time,
-	       uint32_t button, uint32_t state, void *data)
+	       uint32_t button,
+	       enum wl_pointer_button_state state, void *data)
 {
 	struct terminal *terminal = data;
 
 	switch (button) {
 	case 272:
-		if (state) {
+		if (state == WL_POINTER_BUTTON_STATE_PRESSED) {
 			terminal->dragging = 1;
 			input_get_position(input,
 					   &terminal->selection_start_x,