GLSL syntax library

Copperbox revision 457.

Work on the lexer for GLSL, though it is still incomplete.

One odd thing I found is I don't think the GLSL grammar admits the main function, at least not if its signature is

void main (void)
{ ...

I think the grammar would always want a parameter qualifier first after the left paren. To get round this I think I'll have to make main a keyword and give it a special production.