Fix initial video quality blurriness for all codecs

[xianshijing-lk]

Summary

Fixes initial video quality blurriness by:

1. Applying x-google-start-bitrate SDP hint for all video codecs (VP8, VP9, AV1, H264, H265)
2. Defaulting DegradationPreference to MaintainResolution for all video tracks

Problem

Users report video starting blurry for 5-15 seconds before reaching full quality (https://www.loom.com/share/9a34ae565f6949b891a155a436b3d573). This happens because:

1. Slow bandwidth ramp-up: x-google-start-bitrate was only set for VP9/AV1, so VP8/H264 streams start at WebRTC's conservative default (~300 kbps) and ramp up slowly.
2. Resolution degradation: Default DegradationPreference was Balanced, causing the encoder to reduce resolution during the ramp-up period.

Changes

livekit/src/rtc_engine/peer_transport.rs

• Extended x-google-start-bitrate SDP munging to all video codecs (VP8, VP9, AV1, H264, H265)
• Updated tests

libwebrtc/src/rtp_parameters.rs

• Added DegradationPreference enum
• Added set_degradation_preference() and degradation_preference() methods to RtpParameters

livekit/src/room/options.rs

• Added degradation_preference field to TrackPublishOptions
• Added get_default_degradation_preference() that defaults to MaintainResolution
• Re-exported DegradationPreference for users

livekit/src/room/participant/local_participant.rs

• Apply degradation preference to video tracks after creating the transceiver

Behavior
│ Setting │ Before │ After │
│ x-google-start-bitrate │ VP9/AV1 only │ All video codecs │
│ DegradationPreference │ Not set (WebRTC default) │ MaintainResolution │
Users can override degradation_preference in TrackPublishOptions if they prefer Balanced or MaintainFramerate.

Testing

• Added unit tests for DegradationPreference defaults
• Updated x-google-start-bitrate tests for all codecs
• Manual testing with local_video example

[github-actions]

Changeset

The following package versions will be affected by this PR:

Package	Bump
libwebrtc	minor
livekit	minor
livekit-ffi	minor

[boks1971]

LGTM!