Added default IBL mode to IPR Debug Shading

[flat]

Can you add a default simple IBL lighting mode to IPR Debug Shading?

The image lit by the default IBL (dome light) when the user creates a model and selects "Default IBL" in IPR Debug Shading.
(All scene lighting is overridden)

Also, it would be convenient to be able to rotate this Debug IBL (dome light) with Shift right mouse drag.

Useful in Material Look dev.