Fix the subpixel precision offset problem in Color Smudge Paint Op

This patch does two changes:
1) We save not the theoretical hotspot of the lastly painted dab, but
the real center of the dab.
2) Disables the calculation of the subpixel precision values for the dab
in Color Smudge Op. The point is, the Color Smudge Op needs to access
other areas of the image. And this access can be aligned only (yeah,
since non-aligned bitBlt is not possible). So using of subpixel
precision will 1) break alignment; 2) will make the calculation of the
real hotspot of the dab impossible.