gdiplus: don't calculate the number of points in the arc by dividing

[Vincent Povirk]

For bug 19697.

Due to rounding differences, it's possible for the calculated number of points in an arc to be greater than the number of 90 degree sweeps in the arc according to the loop that generates the points. When this happens, that loop will try to generate a section of the arc with a sweep angle of 0, causing a division by 0. Avoid this by using the loop to calculate the number of points.
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