graphics/appleseed: try to unbreak and slightly improve the port (+)

Pull the patch from the Arch/AUR package to unbreak the build against
OSL version 1.13.7 and later. While here, cleanup Imath 3.x support:
drop actually useless patch and fix detection logic in the right file
instead of polluting global CFLAGS and using naive search heuristics.

Nagged by: pkg-fallout (after commit cfcb40e24056)