module: add a 'private' member to expose context

This allows modules that override begin/end_context to supply their
own auxiliary data.

module.c

@@ -8,6 +8,7 @@ module_default_begin_expose(const struct module *mod, cairo_t *cr)
     return (struct module_expose_context){
         .exposable = e,
         .width = e->begin_expose(e, cr),
+        .private = NULL,
     };
 }
 

module.h

@@ -16,6 +16,7 @@ struct module_run_context {
 struct module_expose_context {
     struct exposable *exposable;
     int width;
+    void *private;
 };
 
 struct module {