GlslOptimizerShaderConverter: new plugin

[mosra]

Things to do:

• not sure about the relevance of this nowadays -- with a quick'n'dumb test on a moderately complex shader containing a bunch of loops, those got all unrolled and the resulting SPIR-V binary was significantly larger than without this, even after passing it through SPIR-V optimizer. Or maybe there's a way to limit the unrolling?
• needs to expose input version (same as in glslang), otherwise too shitty to use
• needs to expose preprocessor definitions (~same as in glslang), otherwise too shitty to use
• even with the https://github.com/jamienicol/glsl-optimizer fork which is based on Mesa 20, it reports that the only supported GLSL versions are <= 1.50 and <= 300 es :(
• docs, move the cmake subproject setup to a doc snippet