Workaround image magick issue with greyscale xcf files

IM doesn't seem to properly interpret greyscale xcf files
as being greyscale. Tell it to just take the red channel
in such a case.

Bug: 66323
Change-Id: I46302d43e1029d815be99f481f3942481becd74f
Merge "Workaround image magick issue with greyscale xcf files"