Use a set to track pointer button state.

In addition to `button_count`, we keep track of the current buttons
pressed just as in `wlr_keyboard`.

Add `set_add` and `set_remove` to assist with this. These functions can
only be used with values greater than 0 (such as the button/key masks
for keyboards and pointers).

Partially addresses:
- https://github.com/swaywm/wlroots/issues/1716
- https://github.com/swaywm/wlroots/issues/1593

include/util/array.h

@@ -3,7 +3,22 @@
 
 #include <stdint.h>
 #include <stdlib.h>
+#include <stdbool.h>
 
 size_t push_zeroes_to_end(uint32_t arr[], size_t n);
 
+/**
+ * Add `target` to `values` if it doesn't exist
+ * "set"s should only be modified with set_* functions
+ * Values MUST be greater than 0
+ */
+bool set_add(uint32_t values[], size_t *len, size_t cap, uint32_t target);
+
+/**
+ * Remove `target` from `values` if it exists
+ * "set"s should only be modified with set_* functions
+ * Values MUST be greater than 0
+ */
+bool set_remove(uint32_t values[], size_t *len, size_t cap, uint32_t target);
+
 #endif