graphics/libheif: Correct license, OPTIONize, add jpeg support

Most of this is non-functional change pulling various features out into
options. Part of that is correctly labeling the examples as GPL3+. In a
later revision, they are relicensed to MIT, but not in this version of the
port.

Functional changes including adding a JPEG option and enabling it by
default. libheif is the only utility we ship that can understand .heic
images and converting photos to the more portable jpeg is a reasonable thing
to enable by default.

Approved by: maintainer timeout
Differential Revision: https://reviews.freebsd.org/D18802