Fix shadow rendering for cockpits when raytracing is active

[The-E]

Summary

This branch fixes two problems with player cockpit rendering under Vulkan:

1. Raytraced (RT) shadows on the cockpit showed as solid green.
2. The ship hull could show through the cockpit model, because the two shared one depth buffer.

Fix 1: RT shadows on cockpits

The shadow acceleration structure (TLAS) did not include the cockpit model. It also
did not include OBJ_RAW_POF and OBJ_PROP objects. The shadow ray for the cockpit
pass used the wrong ray origin. In combination, these problems meant that cockpits were
not receiving shadows.

Changes:

• Add the cockpit model to the TLAS gather. Add OBJ_RAW_POF and OBJ_PROP objects
  too. This matches the object set that the rasterized shadow path already uses.
• Give each TLAS instance a ray-cull mask. Tag the viewer's own ship hull with a
  dedicated mask bit. Shadow rays exclude this bit outside the cockpit's own shading
  pass, so the hull does not shadow itself. ship_render_player_ship_casts_shadow_on_cockpit()
  still allows the hull to cast a shadow onto the cockpit during that pass.
• Add a world-space offset to the shadow uniform block. The cockpit render pass uses
  a camera-relative frame, not true world space. The offset corrects the ray origin
  before the shader traces the shadow ray. shadow_cascade_params_bind() now takes
  this offset as a parameter, so the hull draw and the cockpit draw can each supply
  their own value.
• Add shadow_cascade_params_bind_deferred(). This function removes duplicate
  offset-selection code from the OpenGL and Vulkan deferred-lighting passes.
• Add ship_cockpit_render_offset(). This function computes the cockpit's rotated
  and swayed position in one place. The render path, the rasterized shadow path, and
  the TLAS path all call this function. The three paths cannot drift apart.

Fix 2: cockpit depth in Vulkan

OpenGL swaps the depth buffer before it draws the cockpit. The swap keeps the ship
hull's depth values away from the cockpit draw. The Vulkan backend did not do this
swap. The hull's depth values stayed in the depth buffer, so the hull could block
the view of the cockpit.

Changes:

• Add a backup depth image to VulkanPostProcessor.
• Add saveSceneDepth(). This function copies the scene depth into the backup image,
  then clears the live depth buffer for the cockpit draw.
• Add restoreSceneDepth(). This function copies the backup image back after the
  cockpit draw. Later passes that read scene depth, for example the lightshaft pass,
  then read the correct scene depth.

Known gaps

• The deferred lighting pass shades the whole G-buffer in one draw call. The pass
  can apply only one world-space offset per draw. This is correct for the common
  case. It is not correct when Cockpit_shares_coordinate_space is true, or when a
  ship has no cockpit model. This case needs a per-pixel offset, or the offset must
  move into the G-buffer data itself. This branch does not fix this case.
• The cockpit's detail-box check in the TLAS walk uses Eye_position. The
  rasterized path uses a cockpit-relative eye position instead. The TLAS walk skips
  the detail-box check for the cockpit model, rather than compute the correct eye
  position.

Testing

• ninja code and ninja Freespace2 build with no errors.
• Confirmed in-game: player cockpit shadows now render correctly.