Editor: GLSL error feedback & minification

[tschw]

Depends on #6801, adds the last three commits.

[tschw]

Re-pushed: Simplification, a fixed typo and some forgotten whitespace.

[mrdoob]

Thanks thanks!

[benaadams]

btw @zz85 has done a interesting and very useful Emscripten pure js port of glsl-optimizer not what we necessarily want for easy to understand/learning glsl but something to bear in mind

[tschw]

Nice - and it's fast! We should integrate it as a stage before the minifier, which will then declutter the output. Ultimately, both would run when "Publish" is clicked, but we'd need a settings panel to make it all optional.