support 1/2/4 bit palette tiff images with alpha
don't cache vips_system()
vips_system() now uses g_spawn_command_line_sync()

helps stop stray command windows appearing on Windows, better error msg
too
oops, put "%%" squash back in vips_system()

still need this afetr all
Merge branch 'master' into dzsave-gsf

Conflicts:
	ChangeLog