Main change is to not change alpha channel at all. For RGB
channels, calculate the luma value and map the value into 160-224
range. Much simpler and better result.

Change-Id: Ied108c2135f98d78f230a2c0b305bd3396d9ccfd
Reviewed-on: https://gerrit.libreoffice.org/67791
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>