1. Corrected a bug that could lead to IndexError.

2. Corrected the way the coordinates of an image were handled.
Now the pixel coordinates on an image are expected in the order
as any image viewer would show them.

3. Changed the meaning of p, q, r for the shapes of triangles. Now
1/p + 1/q + 1/r < 1 must be satisfied and the angles of the triangle
are pi/p, pi/q, pi/r.

4. If more that 2 image is specified for a tiling, then perfect
alternation is impossible as opposed to what I previously thought, so
corrected the relevant documentation.