we do not need to go via this complex luminance calculation, we
just want the red channel of the RGB data.
This is also what the skia backends do, ever since
commit 2fcfbd73768b69ba58607a054e7f851be2942992
Author: Luboš Luňák <l.lunak@collabora.com>
Date: Fri Apr 3 22:50:12 2020 +0200
no gray conversion needed for VCL alpha hacks
Change-Id: Ie4a7adcc7c488d241ec58e64a130da544c3d39d0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/184944
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins
7f33bccce7