Web: Prune grammar and lexor down to needed subset.

Web: Prune grammar and lexor down to needed subset.

About 60K smaller (20% the target size is these units of 300K).
Over 300 fewer #ifdef.

This adds a new glslang.m4 file that needs to be processed by m4 to get
the glslang.y file needed by bison.  See comment in glslang.m4 for more
detail.

This updates the updateGrammar script to do the .m4 -> .y processing,
to conditionally exclude grammar not needed for the web build.