Zero-sized gizmos were not shown because GPU does not draw filled
triangles with zero area. This problem is resolved by drawing lines or
points in these degenerated cases.
Scaling is supported via computing the original and the new size, then
treat the case with zero original size separately, instead of only
computing the scale itself.
Actually applies to all `cage2d` gizmos, but not compatible when
translate flag is set.
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