gegl: update ctx library.

This change in GEGL goes together with 3 commits on the ctx repository:

• ctx: do not use PATH_MAX. 0001-ctx-do-not-use-PATH_MAX.patch
• src: improve conditional code contents. 0002-src-improve-conditional-code-contents.patch
• Better macro defaults for Windows. 0003-Better-macro-defaults-for-Windows.patch

@ok So I contribute these 3 patches to ctx here, as it doesn't look like there is a dedicated tracker for ctx yet. 🙂

This fixes the GEGL build on Windows, which broke since ctx got introduced. With these changes, ctx still works fine in GEGL (tested both on Linux and Windows builds), i.e. in particular gegl:fill-path operation still works fine.