GMIC: Fix crash when filter is applied to the layer which was moved

- the computation of tile width was buggy
- scaling pixels to float color-space in temporary tile
  did not take into account the posibility that the source
  tile is read only partially (when you e.g. selection)

- there is still some corruption when bitblting
  gmic result to layer

BUG:327980
CCBUG:325771