The result of detecting if a quad should flip the default 0-2 split
when tessellated only used a pre-calculated normal when available,
since the method of detecting the flip was different, the check for a
concave face could change depending on the existence of polygon-normals.
In practice this meant cycles render preview could use a different
tessellation than the GPU display.
While [0] exposed the bug, it's an inherent problem with having 2
methods of detecting concave quads.
Remove is_quad_flip_v3_first_third_fast_with_normal(..) and always
use is_quad_flip_v3_first_third_fast(..), because having to calculate
the normal inline has significant overhead.
Note that "bow-tie" quads may now render with a subdivision in a
different direction although they must be very distorted with both
triangles along the 0-2 split pointing away from each other.
Thanks to @HooglyBoogly for investigating the issue.
[0]: 16fbadde36.
19dbe049db