Update DXR API definitions for final spec. (#659)

* Update DXR API definitions for final spec.

The final version of the DXR API has changed the result type of the `DispatchRaysIndex()` and `DispatchRaysDimensions()` builtins to `uint3` (from `uint2`).

* Add updates for DXR object<->world transformations

The `ObjectToWorld()` and `WorldToObject()` functions were renamed to `ObjectToWorld3x4()` and `WorldToObject3x4()`, resepctively, and then new functions `ObjectToWorld4x3()` and `WorldToObject4x3()` were added to give convenient access to the transpose of these matrices.

(No, I'm not clear on why user's couldn't just call `transpose()`, either)

I've left the old function names in the standard library as forwarding functions just so that we don't break existing DXR code that relied on the old names.