The original version of the icon painter would reach beyong the
texture boundaries for textures of even width and height, and when
painting an icon with the width or height of the texture.
In this case, p3 would be set to the same value as p2. However, the code
expects p3 to lie within the middle rectangle. However, in this corner
case, the middle rectangle is actually empty. So no pixel can lie within
it. Hence, p3 is positioned by one pixel to the left outside the middle
rectangle, which violates the assumptions when calculating the clipping
and texture offsets. The patch fixes the problem by reducing the size of
the right column by one pixel. This ensures that exists always a
non-zero middle rectangle.
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