Non null module callbacks
Wherever modules use the
size_func/prepared_func/updated_func, they need to check that those callbacks are not NULL. This just adds noise to the code.
When modules are called from GdkPixbufLoader, it provides its own such callbacks so they are never NULL.
For other cases where modules are called (the compatibility code which deals with the various
load/begin_load combinations), this branch passes no-op callbacks where NULL was passed before.
This makes the module code slightly cleaner.