snippet: Add a 'replace' string

If present, the 'replace' string will be used instead of whatever code
would normally be invoked for that hook point. It will also replace
any previous snippets.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-snippet-private.h

@@ -44,6 +44,7 @@ struct _CoglSnippet
 
   char *declarations;
   char *pre;
+  char *replace;
   char *post;
 };