Flipped Objects part in book 2 (remove `flip_face` class?)

[oxine]

raytracing.github.io/books/RayTracingTheNextWeek.html

Line 2054 in 5111229

rec.set_face_normal(r, outward_normal);

I' ve noticed that we have already fliped the normal by rec.set_face_normal in hit function, the filp class seems to be useless.We may either remove the set normal function in rect::hit or remove the flip_face class.

[hollasch]

Yow. Excellent comment. A lot of work, but we didn't realize that we'd greatly simplified this part of the code. :)

[hollasch]

Putting this in the v3.1.0 milestone unless we decide otherwise.

[D-K-E]

I can also confirm that I was able to render cornell box without flip_face class and was quite surprised by not having the Empty Cornell box from listing 55.

[hollasch]

Note: see #270 for the original change that implemented proper ray-surface intersection from either side.