update cimg, integrate gmic

try:

$ vips gmic 1 x.jpg 20 1 1 --in0 k2.jpg -- "-blur 10"

output is black, but it does seem to almost work
gmic uses an imagevec for input images
process all gmic images as float
small tidies to vips_gmic.cpp
support uchar and ushort gmic images
add --with-gmic configure option
remove cimg and gmic

move this to a plugin, it's too big for the core library
Merge remote-tracking branch 'origin/master' into update-cimg