Fix bug  #66887 imagescale - poor quality of scaled image

Issue with signed char overflow.

Upstream fix:
https://bitbucket.org/libgd/gd-libgd/commits/4b86e06937bc5ff116be969137f8da9d1a7869d5
NEWS
Merge branch 'PHP-5.5' into PHP-5.6

* PHP-5.5:
  NEWS
  Fix bug  #66887 imagescale - poor quality of scaled image
NEWS