improve GIF edarly close again
improve GIF edarly close again

We were trying to keep the FILE open for gifload between header and
load, but this meant some corrupt GIFs could keep the file open longer
than they should.

Instead, make close into a vfunc and always close between header and
load.

see https://github.com/libvips/libvips/issues/1370#issuecomment-526829415

jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips