Test av_video_hint_alloc with 0, 1, and 4 rects, and
av_video_hint_create_side_data. Verifies that av_video_hint_rects
and av_video_hint_get_rect return pointers consistent with
rect_offset and rect_size, write/read-back of rect coordinates,
both hint type values, and OOM paths via av_max_alloc.
Coverage for libavutil/video_hint.c: 0.00% -> 82.05%
The remaining uncovered lines are the nb_rects overflow guard and
the av_buffer_create / av_frame_new_side_data_from_buf failure
cleanup paths, which av_max_alloc() cannot reach since it forces
the first allocation to fail.
784ab15589