gnu: graphite2: Add fixes for CVE-2017-5436 and other bugs.

* gnu/packages/fontutils.scm (graphite2)[replacement]: New field.
(graphite2/fixed): New variable.
* gnu/packages/patches/graphite2-CVE-2017-5436.patch,
gnu/packages/patches/graphite2-check-code-point-limit.patch,
gnu/packages/patches/graphite2-fix-32-bit-wrap-arounds.patch,
gnu/packages/patches/graphite2-non-linear-classes-even-number.patch:
New files.
* gnu/local.mk (dist_patch_DATA): Add them.