Destroy

I'm unsure about this, we could also do the follow as an extension.

typedef void (*PFN_vit_img_sample_free_cb)(void *user_data);

vit_result_t
vit_tracker_push_img_sample_with_free(vit_tracker_t *tracker, vit_img_sample_t *sample, PFN_vit_img_sample_free_cb cb, void *user_data);