3D: Add origin anchor when placing scenes

[benioriginal]

What problem(s) does this PR solve?

When dragging a 3D scene from the FileSystem dock into the 3D viewport, Godot currently uses the scene's AABB bottom as the placement anchor. This can produce unexpected placement for scenes containing nodes such as Light3D, or for assets whose origin is intentionally positioned for level design.

This implements the improvement proposed in:
godotengine/godot-proposals#13299

Changes

• Preserve the existing AABB-based placement behavior by default.
• Allow holding Ctrl on Windows/Linux, or Cmd on macOS, to place a dragged PackedScene using its origin.
• Allow the origin-placement modifier to be combined with the existing Shift and Alt hierarchy modifiers.
• Document the new modifier in the 3D viewport drop tooltip.
• Keep meshes, materials, textures, and audio on their existing drop behavior.

Usage

When dragging a scene into the 3D viewport:

• No modifier: place using the existing AABB-based anchor.
• Hold Ctrl/Cmd: place the scene origin at the surface hit position.
• Hold Shift: add the scene as a child of the selected node.
• Hold Alt: add the scene as a child of the scene root.

Testing

• Built the Windows editor successfully with MSVC.
• Tested normal AABB-based scene placement.
• Tested Ctrl scene-origin placement.
• Tested combining Ctrl with Shift and Alt.
• Tested that mesh and other resource drops retain their existing behavior.
• Verified formatting and git diff --check.

AI disclosure

AI assistance was used for proofreading the contribution text.

2026-07-23.14-56-32.mp4

• Bugsquad edit, closes: Allow dragging scenes from the FileSystem dock to use their origin instead of their AABB as anchor godot-proposals#13299