Add an Appendix/Wiki page for using a GPU debugger

[armansito]

Add a basic guide for running Xcode and RenderDoc to step through the shader code, inspect resources, etc. Launching the Rust binary with Xcode (when running with the wgpu Metal backend on macOS) is very easy. I haven't tried debugging the example code with RenderDoc on Windows / Linux though.

The appendix can use the NaN propagation bug in the refraction chapter as an example. It should go through the process of launching the debugger and walk through it step-by-step and get to the root cause. For example:

Radiance Texture and Framebuffer:

Where NaN is produced (0/0 in normalize):

Because refract returns a null ray:

@trevordblack

[trevordblack]

If you want I'm happy to try and step through this in renderdoc on linux and PIX on windows (haven't used PIX in any serious capactiy, now is as good an opportunity as any).

Any idea when you're gonna want this?

[armansito]

Thank you @trevordblack, that would be very helpful. It's not urgent so please don't feel pressured.