Book 3.4: Update sphere importance sampling f(x)

[rupsis]

Addresses #1500

[rupsis]

@dimitry-ishenko Might want to take a peek as well.

[hollasch]

Sorry for the delay in giving feedback here. This is tightly coupled with Dimitry's issue #1536, which I believe should also be included in this change. I'm working on broadening this change to include #1536 (two paragraphs up), which covers the mapping of a point on the unit sphere to its two polar angles.

[rupsis]

Sorry for the delay in giving feedback here. This is tightly coupled with Dimitry's issue #1536, which I believe should also be included in this change. I'm working on broadening this change to include #1536 (two paragraphs up), which covers the mapping of a point on the unit sphere to its two polar angles.

Sounds good. Want to take what I have here and roll them into your changes?

If so, feel free to close the PR out.

[hollasch]

I'm just working in this branch.

[hollasch]

I'm going to add my changes onto your own.