[gzip] Update sources to zlib 1.2.11

This can be tested by building with the Unix development build

  make setup devel

or by building the freetype-demos programs with

  meson setup build -Dfreetype2:zlib=internal
  meson compile -C out

and trying to run `ftview` with a `.pcf.gz` font file.

* src/gzip/ftgzip.c, src/gzip/rules.mk: Update for new zlib sources.  Also
remove the temporary fix introduced in commit 6a431038 to work around the
fact that the internal sources were too old.

* src/gzip/README.freetype: New file describing the origin of the sources
and how they were modified.

* src/gzip/patches/*: Patch files applied to original sources.

* src/gzip/*: Updated zlib sources with the patch file(s) from
`src/gzip/patches/` applied, followed by a conversion with zlib's
`zlib2ansi` script.
26 files changed