Use sentinels instead of special casing first and last stops

Use sentinels instead of special casing first and last stops

When storing the gradient stops internally, allocate two more stops,
one before the beginning of the stop list and one after the
end. Initialize those stops based on the repeat property of the
gradient.

This allows gradient_walker_reset() to be simplified because it can
now simply pick the two closest stops to the position without special
casing the first and last stops.