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/gears.c b/clients/gears.c
index 5cfc298..166e6a4 100644
--- a/clients/gears.c
+++ b/clients/gears.c
@@ -248,12 +248,13 @@
 
 static void
 button_handler(struct widget *widget, struct input *input,
-		uint32_t time, uint32_t button, uint32_t state, void *data)
+		uint32_t time, uint32_t button,
+		enum wl_pointer_button_state state, void *data)
 {
 	struct gears *gears = data;
 
 	if (button == BTN_LEFT) {
-		if (state) {
+		if (state == WL_POINTER_BUTTON_STATE_PRESSED) {
 			gears->button_down = 1;
 			input_get_position(input,
 					&gears->last_x, &gears->last_y);