-Base class for things that can store data in buffers. Supports buffer sharing.
-A dirty flag is provided for derived classes. It should be set when the data
-in the buffer is considered out of date, and is cleared by Bufferable after
-uploading fresh data to the buffer.
+Base class for things that can store data in buffers. Multiple Bufferables
+may be put in the same buffer.
+
+Derived classes should call mark_dirty() when the stored data has changed.