This regression came from 09bb5cc3d3676e88048d5cd845b075ea30ca8e66 as
now we don't Sprite::setTransparentColor() on each undo/redo and only
when needed. This brought a new kind of error where the mask color for
images was set to -1 after convert_pixel_format().
This also fixes a conversion from Indexed -> RGB where the transparent
color was not set back to 0. And the transparent color is always set
when we are in indexed mode to avoid any assert in debug mode.
ec42689b82