Fix shadow map debug visualization camera frustum index buffer size

[stoofin]

#81288 changed to 16-bit indices but missed a spot updating buffer sizes, so using Display Advanced... -> Directional Shadow Map doesn't show the camera frustum and fills Output with the following error in 4.2.beta2:

  Default index buffer initializer array size (144) does not match format required size (72).
  drivers/vulkan/rendering_device_vulkan.cpp:4584 - Condition "!index_buffer_owner.owns(p_index_buffer)" is true. Returning: RID()
  Default index buffer initializer array size (96) does not match format required size (48).
  drivers/vulkan/rendering_device_vulkan.cpp:4584 - Condition "!index_buffer_owner.owns(p_index_buffer)" is true. Returning: RID()
  drivers/vulkan/rendering_device_vulkan.cpp:7408 - Parameter "index_array" is null.
  Draw command requested indices, but no index buffer was set.
  drivers/vulkan/rendering_device_vulkan.cpp:7408 - Parameter "index_array" is null.
  Draw command requested indices, but no index buffer was set.

[akien-mga]

Thanks!