SDL_gfx: 2.0.25 -> 2.0.26

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:
- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.0.26 with grep in /nix/store/cbvm4r21kf3acf43hff5sf6r1vbd6lpg-SDL_gfx-2.0.26