nsgifload: enforce maximum GIF dimensions of 16383
nsgifload: enforce maximum GIF dimensions of 16383

The GIF spec allows dimensions up to 65535 (16-bit unsigned),
but this equates to 17GB/frame.

A common task is to convert animated GIF to animated WebP,
and the latter supports dimensions up to 16383 (14-bit unsigned),
so that seems like a sensible limit and equates to 1GB/frame.

Also makes the error message consistent with other loaders.

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