Fix missing initial frames and seeking in MPEG-TS/M2TS files

[rajeshgk]

Summary

Fix frame-accurate reading and seeking of MPEG-TS/M2TS (AVCHD) video files in FFmpegReader.

Addresses #1031.

Problem

When importing MPEG-TS/M2TS files into OpenShot, the first 25–30 frames can be missing or delayed during clip and timeline preview.

Remuxing the same video to MP4 without re-encoding avoids the issue, indicating a container-specific timestamp and seeking problem.

Changes

• Correct the frame-to-video-PTS conversion to account for the initial timestamp offset.
• Correct working-frame timestamp alignment.
• Index MPEG-TS video keyframes while reading packets.
• Use indexed byte positions for MPEG-TS seeking when a suitable keyframe is available.
• Preserve the keyframe index across internal reader reopens.
• Reset the index when opening a fresh input.
• Apply the larger 90-frame seek buffer only to MPEG-TS, preserving the existing 12-frame buffer for other formats.
• Discard retained packets before repositioning the demuxer.

Validation

• 29/29 FFmpegReader C++ regression tests passed.
• 28/28 PNG SHA-256 frame comparisons passed.
• Tested MPEG-TS and MP4 files with GOP lengths of 90 and 180 frames.
• Verified frames 1, 30, 90, 180, 270, 300, and 359.
• Verified seeking to a later indexed MPEG-TS keyframe.
• git diff --check passed.

Scope

Changes are limited to:

• src/FFmpegReader.cpp
• src/FFmpegReader.h