Change gimp_viewable_get_[new]_preview() to return buffers of the
image's and layers' colorspace, but in u8. This way layer and image
previews can transform them correctly to the display profile.

Note: this makes plug-ins receive thumbnail buffers in that
pixel format too.

Also change gimp_viewable_get_[new]_pixbuf() to always return sRGB
buffers that can reasonably be put to screen directly, or put into DND
buffers. This is at least more correct now.