glslviewer: 2019-04-22 -> 1.6.8, use python3

[jonringer]

Motivation for this change

bump to latest stable

wrap using python3 instead of 2, had to run 2to3 to get it to be valid python3 code.

issue upstream: patriciogonzalezvivo/glslViewer#191

related: #101964

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.