obs-ffmpeg: Add NVENC preset mapping for old Default preset

The old NVENC preset "Default", labeled in OBS as "Performance", maps to
the new P3 preset when not using multipass. Since we previously only
enabled multipass/two-pass on "Max Quality", we can map this to P3 in
all cases.

This commit also modifies UI.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f