Extract collision detection into a ui module

src/ui.c

@@ -0,0 +1,6 @@
+#include "ui.h"
+
+bool aabb(const rect_t *rect, i32 x, i32 y) {
+  return x > rect->topleft.x && x <= rect->topleft.x + rect->w &&
+         y > rect->topleft.y && y <= rect->topleft.y + rect->h;
+}