Set color table fourth channel to zero for 1 and L mode when saving BMP

[radarhere]

Resolves #8888

https://www.ece.ualberta.ca/~elliott/ee552/studentAppNotes/2003_w/misc/bmp_file_format/bmp_file_format.htm states that the fourth channel in a BMP color table is 'unused (=0)'.

It seems reasonable to do so when saving 1 and L mode BMPs, and makes more intuitive sense than our current strategy of setting the fourth channel to the same value as the other three.