feat: VideoTrackRender.placeholderBuilder

[bdlukaa]

Fixes #1061

Example usage:

VideoTrackRenderer(
  ...,
  placeholderBuilder: (context) {
    return const CircularProgressIndicator.adaptive();
  },
)

[Copilot]

Pull request overview

Adds a new placeholderBuilder API to VideoTrackRenderer to allow callers to show a custom widget while the underlying RTCVideoView is still initializing / not yet rendering frames (addressing the “black view during setup” UX gap described in #1061).

Changes:

• Introduces VideoTrackRenderer.placeholderBuilder (optional WidgetBuilder) as a new public parameter.
• Wires placeholderBuilder through to flutter_webrtc’s RTCVideoView in both web and native render paths.
• Adds a .changes entry documenting the new capability.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/src/widgets/video_track_renderer.dart	Adds the placeholderBuilder API and forwards it into RTCVideoView.
.changes/track-render-placeholder	Adds a changelog fragment for the new placeholder capability.

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[bdlukaa]

Related: flutter-webrtc/flutter-webrtc#2102