Objective

• Even after 2D picking with a 3D camera #962, 2D picking only works reliably when pickable colliders are located on the Z = 0 plane when e.g. using perspective projection or a rotated camera. Allowing this Z coordinate to be configured could be useful.

Solution

• Make the Z level used for picking ray intersection configurable by adding a new field to PhysicsPickingSettings.
• Extend the picking_2d_in_perspective example with controls for moving the physics picking Z plane, rotating the camera, and switching projection type, demonstrating this new functionality.
• Write a migration guide entry for the changes in this PR and 2D picking with a 3D camera #962.

Testing

• The extended picking_2d_in_perspective example seems to work properly.
• The solution works in my use case, picking from a 3D camera with perspective projection and colliders located far away in negative Z, which was previously broken.
• cargo test returns 4 failures, but I get the same result on the latest commit before my changes.

  ---- dynamics::rigid_body::mass_properties::tests::mass_properties_add_remove_collider stdout ----
  
  thread 'dynamics::rigid_body::mass_properties::tests::mass_properties_add_remove_collider' (22023) panicked at crates/avian2d/../../src/dynamics/rigid_body/mass_properties/mod.rs:736:9:
  assertion `left == right` failed
    left: 15.707963
   right: 15.707964
  
  ---- dynamics::rigid_body::mass_properties::tests::mass_properties_no_auto_mass_add_remove stdout ----
  
  thread 'dynamics::rigid_body::mass_properties::tests::mass_properties_no_auto_mass_add_remove' (22026) panicked at crates/avian2d/../../src/dynamics/rigid_body/mass_properties/mod.rs:890:9:
  assertion `left == right` failed
    left: 14.999999
   right: 15.0
  
  ---- dynamics::rigid_body::mass_properties::tests::mass_properties_move_child_collider stdout ----
  
  thread 'dynamics::rigid_body::mass_properties::tests::mass_properties_move_child_collider' (22025) panicked at crates/avian2d/../../src/dynamics/rigid_body/mass_properties/mod.rs:843:9:
  assertion `left == right` failed
    left: 14.999999
   right: 15.0
  
  ---- dynamics::rigid_body::mass_properties::tests::mass_properties_rb_collider_with_set_mass_and_child_collider_with_set_mass stdout ----
  
  thread 'dynamics::rigid_body::mass_properties::tests::mass_properties_rb_collider_with_set_mass_and_child_collider_with_set_mass' (22033) panicked at crates/avian2d/../../src/dynamics/rigid_body/mass_properties/mod.rs:629:9:
  assertion `left == right` failed
    left: 14.999999
   right: 15.0
  

All testing done was on x86_64-unknown-linux-gnu (latest CachyOS Desktop as of writing, using the rust package from the cachyos-extra-znver4 distribution repository).