GLib now opt-in dependency; transceiver mid() no longer aborts

Fixes

• Fix silent subscription failures in single-pc mode when the SFU reuses an existing empty transceiver for a new remote track. Also make RtpTransceiver::mid() safe to call on transceivers that haven't been negotiated yet — libwebrtc is built with -fno-exceptions, so std::optional::value() aborted the process instead of throwing.
• Add LK_DISABLE_NVDEC to bypass NVIDIA NVDEC decoder registration when the environment variable is set.
• Add Jetson DMA-buffer video publishing support for libargus MIPI capture and the Jetson hardware encoder, including AV1 hardware encoding on supported Jetson Orin devices.

Make GLib an opt-in dependency

webrtc-sys no longer links against glib-2.0/gobject-2.0/gio-2.0 by default.

Breaking: Wayland screen sharing now requires the glib-main-loop feature on livekit (or libwebrtc).