A base class for filters that utilize separate Objects as a masking object.
Not intended to be used directly.
Masking objects have a number of restrictions on them, see
AbstractMask.mask for details.
The Object contents of which serve as a mask to the filtered Object.
Masking Objects have following limitations: It cannot be a parent of the filtered Object. It should not contain any filters. It should be present in the object tree and precede the Object it masks in the rendering order (rendered before it). Same masking Object cannot be used by multiple mask filters.
When enabled, masking Object will be visible to the user. Hidden otherwise. ( default : false )