feat: add custom video source.

[cloudwebrtc]

feat: custom video source via RTCVideoSource::OnCapturedFrame

Let's applications inject their own video frames into the libwebrtc pipeline
without going through a camera or desktop capturer. Adds a wxWidgets demo
that uses the new API to stream synthetic SMPTE colour bars between two
in-process peer connections.

Public API additions

• RTCVideoSource::OnCapturedFrame(scoped_refptr<RTCVideoFrame>)
• RTCVideoSource::GetSourceType()
• RTCPeerConnectionFactory::CreateCustomVideoSource(string video_source_label, scoped_refptr<RTCMediaConstraints> constraints)

Implementation notes

• internal::VideoCapturer::OnFrame promoted from protected to public
  so RTCVideoSourceImpl can forward user frames into the broadcaster
  without a friend declaration.
• RTCVideoSourceImpl gains a second constructor that stores a
  shared_ptr<internal::VideoCapturer>; the desktop-source path keeps the
  old single-arg ctor and treats OnCapturedFrame as a no-op.
• RTCVideoFrame::timestamp_us == 0 falls back to webrtc::TimeMicros() so
  callers can omit the timestamp.

Demo: examples/wx_loop_back_demo

• wxWidgets app (CMake + find_package(wxWidgets)).
• Two in-process RTCPeerConnections, full offer/answer + ICE exchange,
  one audio track and one synthetic video track.
• Frame producer pushes 640×480 I420 SMPTE colour bars scrolling at
  ~120 px/sec via OnCapturedFrame.
• Decoded remote video rendered into a wxPanel (I420 → ABGR via
  libyuv → wxImage → wxBitmap).
• macOS bundling: post-build copies libwebrtc.dylib into
  Contents/Frameworks/, rewrites the executable's recorded dependency to
  @rpath/libwebrtc.dylib, ad-hoc re-signs both binaries.
• README with build, run, and verification steps.