glsl: Defer initialization of built-in functions until they're needed.

Very simple shaders don't actually use GLSL built-ins.  For example:
- gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
- gl_FragColor = vec4(0.0);
Both of the shaders used by _mesa_meta_glsl_Clear() also qualify.

By waiting to initialize the built-ins until the first time we need to
look for a signature, we can avoid the overhead entirely in these cases.

Makes piglit run roughly 18% faster (255 vs. 312 seconds).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/glsl/ast_function.cpp

@@ -117,6 +117,7 @@ match_function_by_name(exec_list *instructions, const char *name,
       /* The current shader doesn't contain a matching function or signature.
        * Before giving up, look for the prototype in the built-in functions.
        */
+      _mesa_glsl_initialize_functions(state);
       for (unsigned i = 0; i < state->num_builtins_to_link; i++) {
 	 ir_function *builtin;
 	 builtin = state->builtins_to_link[i]->symbols->get_function(name);