cbs: Describe allocate/free methods in tabular form

Unit types are split into three categories, depending on how their
content is managed:
* POD structure - these require no special treatment.
* Structure containing references to refcounted buffers - these can use
  a common free function when the offsets of all the internal references
  are known.
* More complex structures - these still require ad-hoc treatment.

For each codec we can then maintain a table of descriptors for each set of
equivalent unit types, defining the mechanism needed to allocate/free that
unit content.  This is not required to be used immediately - a new alloc
function supports this, but does not replace the old one which works without
referring to these tables.

libavcodec/cbs.h

@@ -349,6 +349,15 @@ int ff_cbs_alloc_unit_content(CodedBitstreamUnit *unit,
                               size_t size,
                               void (*free)(void *opaque, uint8_t *content));
 
+/**
+ * Allocate a new internal content buffer matching the type of the unit.
+ *
+ * The content will be zeroed.
+ */
+int ff_cbs_alloc_unit_content2(CodedBitstreamContext *ctx,
+                               CodedBitstreamUnit *unit);
+
+
 /**
  * Allocate a new internal data buffer of the given size in the unit.
  *