- disable auto-adding of the lcms display filter module

- change profile convert dest formats to be always R'G'B'A, a display
  profile transform outputs something that can be displayed directly,
  so no additional gamma transform must happen when the pixels are
  copied to a cairo-ARGB32 buffer

- add a medium forest of if() branches to gimpdisplayshell-filter.c
  which cover all combinations of profile and display filter
  transforms

- all of this is still very broken when changing an image to linear,
  because the configured RGB profile from prefs will do horrible
  nonsense (things work fine though with a per-image profile that is
  for linear data)