Definition of a bend: ends of the line should always be bends, otherwise not
all line vertices are covered by bends (definition elsewhere).

Gentle inflection at the end of the bend: the article does not specify how many
vertices should be included when calculating the end-of-bend inflection. We
chose the iterative approach -- as long as the angle is "right" and the
distance is (greedily) decreasing, keep going.

Self-line crossing when cutting a bend: the self-line-crossing may happen
after a few bends have been skipped. E.g. ends of A<->B cross the line, but
"swallow" a few more in between:

   ,______
  /       \
  |___A   |    \  |
      \   |    B\ |             __
       \  |     | |            /  \
       /  |     | |___,---,___/A  |
      /   |     \_________________|
      \   |
       \  |
       /  /
  ----/  /
 / ,____/
/ B|
   |

If a bend with 180+ deg inflection is found, its line between inflection angles
(AB in our examples) must be crossed with all the other bends to detect a
possible line-crossing. This is O(N*M), where N is the total number of line
segments, and M is the number of qualifying bends. In other words, can be very
computationally expensive.

This may be slightly computationally simplified: if other bend's
endpoints (A' and B') are in different sub-planes as divided by AB, then the
crossing exists, and more expensive st_split can be used.