redirect OpenSlide error logging to vips_error()
redirect OpenSlide error logging to vips_error()

OpenSlide uses g_critical() and g_warning() for unexpected conditions,
so if these are configured as fatal errors in glib (e.g. libvips or
nip2 is built with --enable-debug=yes) the program will crash instead.

mcanthony/libvips,leiyangyou/libvips,mcanthony/libvips,wishabi/libvips,wishabi/libvips,leiyangyou/libvips,mcanthony/libvips,lovell/libvips,lovell/libvips,leiyangyou/libvips,leiyangyou/libvips,jcupitt/libvips,jcupitt/libvips,wishabi/libvips,jcupitt/libvips,mcanthony/libvips,lovell/libvips,lovell/libvips,wishabi/libvips,jcupitt/libvips