Include file:
When a tree of graphical components is saved to disk (translated to ASCII), it is
possible to enable the output of common composite-graphics as 'pic' records,
to allow for compression of replicated composite-graphics within a single document.
When reading these documents back in, the istream constructor of OverlayIdrawComp
(and other top-level components) uses this mixin to manage a table of pics,
building it up as each 'pic' record is read, and accessing entries for each
":pics id" field found in subsequent records.