- Remove handles as users should use the Generate Visibility AABB
option whenever possible, so manual resizing isn't needed often.
(For particles that move quickly and have Local Coords disabled,
Extra Cull Margin can be increased in the GeometryInstance3D
properties in the inspector.)
- Remove translucent filled part of the gizmo to be more consistent
with GeometryInstance3D's recently introduced Custom AABB gizmo.
- Remove center marker as it can visually obstruct particles.
This paves the way for future emission shape gizmos which will
feature handles.
