fix extra band handling in colour

backport of new colour.c from 7.41
Merge branch '7.40'

Conflicts:
	ChangeLog
	configure.ac
	libvips/colour/Lab2LabQ.c
	libvips/colour/Lab2LabS.c
	libvips/colour/LabS2Lab.c
	libvips/colour/LabS2LabQ.c
	libvips/colour/float2rad.c
	libvips/colour/sRGB2scRGB.c
	libvips/colour/scRGB2sRGB.c