fixes, some important

* contrib/hbhpdf/3rd/libhpdf/hpdfimap.c
  * contrib/hbhpdf/3rd/libhpdf/libhpdf.dif
    + patched to build with libpng 1.6 w/o warning
      identical to this upstream patch:
         https://github.com/libharu/libharu/commit/f1817baea49ab09de4635e383dc86b04a0fc3d8f

  * bin/commit.hb
    ! fixed to add shebang to newly created commit hook

  * bin/check.hb
    ! exclude maskimag.png from processing
    ! typo in png processor name
    ! minor syntax clarification

  * contrib/hbhpdf/tests/files/maskimag.png
    ! restored original version, as any optimized version will
      cause libharu to GPF (visible when running the demo)