Use yy_scan_string and stop caring about shader->SourceLen.

We had to call strlen on the preprocessed source, which seemed a bit
pointless; also, we updated shader->SourceLen but not shader->Source,
which was even more confusing.  Just leave both untouched.
diff --git a/glsl_parser_extras.h b/glsl_parser_extras.h
index cad3424..1edd86b 100644
--- a/glsl_parser_extras.h
+++ b/glsl_parser_extras.h
@@ -102,11 +102,11 @@
 			       const char *fmt, ...);
 
 extern "C" {
-extern int preprocess(void *ctx, const char **shader, size_t *shader_len);
+extern int preprocess(void *ctx, const char **shader);
 }
 
 extern void _mesa_glsl_lexer_ctor(struct _mesa_glsl_parse_state *state,
-				  const char *string, size_t len);
+				  const char *string);
 
 extern void _mesa_glsl_lexer_dtor(struct _mesa_glsl_parse_state *state);