Log Message:
jbig2dec: Update to 0.18

pkgsrc changes:
 - Remove patches/patch-autogen.sh, applied upstream
 - Remove not needed GITHUB_TAG reinitialization, it is already initialized to
   ${PKGVERSION_NOREV} by default

Changes:
0.18
----
 * Performance enhancements related to decoding of MMR and generic
   regions as well as composing images onto pages.
 * Bug fixes for a few issues reported by Coverity and OSS-Fuzz.