BitmapColor itself is kept to distingish the Color usage as part
of a color palette, which continues to store the offset in the
blue value. The original special mbIndex handling is long gone
since commit 1fefdd6f ("Alpha channel in BitmapColor - change
bIndex to alpha"), so there is no data difference.

This also results in the following changes:

* now has a basic_ostream<charT, traits>& operator<<
  (that was my actual starting point... for an other bug fix)
* there is a minimal difference for GetLiminance
  BGR(29,151,76) => BGR(28,151,77)
* no more return values for Merge and Invert
  (previously returning *this)
* replaces all GetBlueOrIndex with GetIndex

This leaves one "problematic" part: the GetColorError handling.
At first glance it should probably be virtual. The Color variant
is less strict then the BitmapColor one - for whatever reason.
BitmapColor is always used to search for the best match in a
Palette. Currently I'm simply leaving both variants. Would be
nice to have an explict for functions here.

Change-Id: I251ba3024a1d60f2a9d9fde9cd0a60f08e8322a7
Reviewed-on: https://gerrit.libreoffice.org/72181
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>