obs-outputs: File splitting improvements for Hybrid MP4/MOV

[derrod]

Description

Fixes file splitting by ensurnig the last frame of each part is not dropped, and b-frames are accounted for in the timer

Motivation and Context

Address part of #13374.

Originally, before file splitting was introduced, I made the choice to simple ignore the last frame in the packet queue of the muxer, because frame N+1 is required to properly determine the duration and I figured nobody would notice if their recording is one frame short.

However, when using frame splitting this would result in the last frame before a split being dropped. To fix this, we can simply reuse the previous sample's duration for the final sample in a file. Since OBS only does CFR, they should all be identical anyway.

Additionally I discovered while testing that the first split would always be one GOP too long (e.g. 1m2s instead of 1m) due to the fact that b-frames and negative start DTSes were not accounted for. This is also fixed in this PR.

How Has This Been Tested?

Recorded some files, looked at them frame-by-frame to confirm none are missing now.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.