Create two separate functions to add tiles in the odd and even sequences

At this point, both functions have the same implementation, copied as is
from the function addTileAndSymmetricTiles() previously used for both the
odd and the even sequences.

The goal of this ongoing refactoring is to remove the gap between the
four quarters of the circles drawn with the even sequence.

The circle sector shall no longer be shifted for the odd circle, and the
computation of positions for symmetric points shall be customized.
Use unshifted y value in computation of getEvenWidth()

This implementation now uses the formulae described in the prototype:
  2 * y + 2
or
  2 * (y + 1)
Shift x/y values for odd sequence in addEvenSequenceTiles()

No shift occurs in the calling context anymore. I removed the variables
xOdd/yOdd and xEven/yOdd from the calling context, using simply x/y instead.
Fix computation of symmetrical positions in odd circle sequence

In this updated computation, x and y are replaced with xSym = x + 1
and ySym = y + 1 in some places. The list of points inserted at each
step becomes:

1: x, y
2: -xSym, -ySym
3: -xSym, ySym
4: x, -ySym
5: -ySym, x
6: y, -xSym
7: -ySym, -xSym
8: y, x

Also, I removed the checks for tiles on the vertical and horizontal axes
of symmetry, as well as their intersection point, which never occurs
in the odd circle sequence: tiles are added on each side of these axes,
never on the axes themselves, which lie at the edge of tiles.
Fix vertical position of odd circles

The center position was shifted 1 tile down.