GeglMetadataStore_declare

The _declare virtual method creates a metadata variable in the underlying data store. It implements gegl_metadata_store_declare(). A GParamSpec is used to describe the variable. If the metadata shadows an object property, shadow should be TRUE, otherwise FALSE. It is acceptable for a subclass to provide additional variables which are implicitly declared, that is, they need not be declared using gegl_metadata_store_declare(), however the pspec method must still retrieve a GParamSpec describing such variables. This method MUST be provided by the subclass.