obs-ffmpeg: Increase initial buffer fullness for VAAPI

[nowrep]

Default ffmpeg initial buffer fullness (75%) is too low and results in significantly lower video quality on AMD cards.

Changing it to 100% fixes the quality and also matches what AMF encoder is doing.

Description

Set HRD initial buffer fullness to 100% to fix video quality on AMD cards.

Motivation and Context

Encoded video quality on AMD cards is rather bad with default settings at low bitrates (6000kbps).

How Has This Been Tested?

Tested on ArchLinux with mesa 23.1.3 and 7900XTX.

Types of changes

• 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.