shaders: parametrize/abstract ident_t v3 (unsigned short)

Supersedes: !413 (closed)

From some quick testing, this reduces CPU footprint of shader generation by anywhere from 10% to 50% (depending on the shader), when -DPL_STRIP_SHADERS is set

TODO:

• bikeshed $ name - this is not technically a legal identifier but gcc/clang/etc. accept it, so I'm happy using this during the transition period
• expose PL_STRIP_SHADERS as an option not guarded by NDEBUG