Compiler extensions

Can we have extension support to the compiler? I currently want the compiler to store some meta data to make pipeline creation simpler. I'd rather have some extension I can add to the compiler than have to muck about its guts.

Start by reviewing the compiler extension request and how compiler state is currently exposed in glslang. Determine the extension boundary and metadata lifecycle before proposing an implementation. Done means a documented extension mechanism supports pipeline metadata without requiring changes to compiler internals.

Written by the indexing model from the issue text.