glsl2: talloc the glsl_struct_field[] we use to look up structure types.

Since the types are singletons across the lifetime of the compiler,
repeatedly compiling a program with the same structure type defined
would drop a copy of the array on the floor per compile.

This is a bit tricky because the static GLSL types are not called with
the talloc-based new, so we have to use the global type context, which
may not be initialized yet.

src/glsl/ast_to_hir.cpp

@@ -2471,8 +2471,8 @@ ast_struct_specifier::hir(exec_list *instructions,
     * the types to HIR.  This ensures that structure definitions embedded in
     * other structure definitions are processed.
     */
-   glsl_struct_field *const fields = (glsl_struct_field *)
-      malloc(sizeof(*fields) * decl_count);
+   glsl_struct_field *const fields = talloc_array(state, glsl_struct_field,
+						  decl_count);
 
    unsigned i = 0;
    foreach_list_typed (ast_declarator_list, decl_list, link,