Add drawing buttons

src/button.c

@@ -0,0 +1,10 @@
+#include "button.h"
+#include "window.h"
+
+#define BUTTON_FILL_COLOUR ((colour_t){.abgr = 0xff89a83c})
+#define BUTTON_BORDER_COLOUR ((colour_t){.abgr = 0xff768432})
+
+void draw_button(const window_t *wnd, const button_t *button) {
+  fill_rect(wnd, &(button->rect), BUTTON_FILL_COLOUR);
+  draw_rect(wnd, &(button->rect), BUTTON_BORDER_COLOUR);
+}