Frame accessing from H.265(HEVC) videos issues

I get different frames count when using decord and opencv to access information from HEVC videos. Also, when I try to load certain segment from video, the clip is totally different compared to moviepy which receives time stamp directly.

`vr = de.VideoReader(video_path)`
`cap = cv2.VideoCapture(video_path)`
`num_frames = cap.get(cv2.CAP_PROP_FRAME_COUNT)`
`if int(num_frames) != len(vr):`
` print(num_frames)`
` print(len(vr))`

Start by reproducing the frame-count comparison between decord's VideoReader and OpenCV's cv2.VideoCapture on the reported H.265/HEVC videos. Compare the segment selected by decord with moviepy's timestamp-based result, then determine the expected frame count and clip boundaries; the issue is resolved when these behaviors are consistent or the codec limitation is documented.

Written by the indexing model from the issue text.