feat(snapshot): add avatars-only capture mode

[shadoskill]

Renders avatars and their attachments on a transparent background (save as PNG to keep transparency). Four opt-in sub-options:

• Include seat objects (SnapshotSitObjects)
• Include animesh (SnapshotAnimesh)
• Include particles (SnapshotParticles, attachment-sourced only)
• Include selected objects (SnapshotSelectedObjects)

World geometry, sky, water, and terrain are culled. Particle groups are filtered to those whose emitter source is an avatar attachment, preventing world emitters from corrupting the transparent background. Seat and selected objects use makeActive() to move into spatial bridges so the avatars-only cull filter passes them; a depth-stamp pass writes their silhouette into the alpha channel.

Description

This add the ability to take screen shots with a transparent background only showing avatars, animesh, particles from the avatar, objects sat on, and selected objects.

This will make it easier for users to take screen shots of just avatars and specific objects without having to deal with the drawbacks of a green screen.

---

Checklist

Please ensure the following before requesting review:

• I have provided a clear title and detailed description for this pull request.
• If useful, I have included media such as screenshots and video to show off my changes.
• The PR is linked to a relevant issue with sufficient context.
• I have tested the changes locally and verified they work as intended.
• All new and existing tests pass.
• Code follows the project's style guidelines.
• Documentation has been updated if needed.
• Any dependent changes have been merged and published in downstream modules
• I have reviewed the contributing guidelines.

---

Additional Notes