obs-outputs: Fix path normalization logic

[grimfoss]

Description

Changed order of operations in generate_filename() in both mp4-output.c and obs-ffmpeg-mux.c so that the full file path gets normalized to use forward slashes, instead of just the directory.

Motivation and Context

In the event that the user provides a formatting string with windows-style directories (%MM\%DD\%hh-%mm-%ss), encoding would either fail to open a new file or crash entirely if the file would be split after the date changes, due to the logic for making a new directory during file splitting only looking for the last forward slash.

How Has This Been Tested?

To test, I set automatic file splitting to 1 minute, and made the formatting string "%ss\%mm" so every split was guaranteed to make a new directory. Running a recording for one minute in each format no longer resulted in a crash or failure at the split, and a new directory being created was successfully observed in each test.

Testing environment:
Windows 11 Home 10.0.26200
AMD Ryzen 9 7900x Processor
Nvidia RTX 5070 Gpu
32GB GDDR5 Memory

Testing was done manually, and I do not expect these changes to affect any other areas.

Types of changes

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

Tweak (non-breaking change to improve existing functionality)

Checklist:

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

[grimfoss]

Background if there are issues: I'm a CS student who's only taken some intro classes so far, I ran into this issue and tracked down the cause pretty quickly and wanted to try my hand at fixing it. I'm not incredibly familiar with git but I believe I followed everything to a tee. Never contributed to anything before so any feedback on any part of this would be greatly appreciated!