Sort tiles in the reference circle sector by distance and x

Step C of the algorithm is now complete:

  C) collect tiles [distance, x, y] from the circle sector
     and sort them by ascending distance then by ascending x
     (which is also by descending y).
Capitalize start of sentences in comments
Move comment and initialization step of step C of the algorithm together

The goal is to keep related information in close relationship.
Add declaration of arrays for even and odd tile sequences
Reorder x and y in list of declarations
Start adding tiles by symmetry
Simplify, correct and complete computation of even/odd tile sequences

This completes step D of the algorithm:

  D) insert extra tiles by symmetry to create two sequences
     for even and odd circles