RFC-24: published implementation proposal on bugzilla

[Umberto Nicoletti]

I have been working on an example implementation of rfc-24 and the
fruit of my labor is attachment #632 to:
http://mapserver.gis.umn.edu/bugs/show_bug.cgi?id=2032

Direct link:
http://mapserver.gis.umn.edu/bugs/attachment.cgi?id=632&action=view

Apply this patch to cvs HEAD to see how rfc24 would be implemented.
The patch only modifies the map and layer objects, the rest is still unmodified.

Key points:
1. refactor map->layers to be an array of pointers instead of a array of structs
2. allocation of a new layerObj only happens when needed, thus conserving memory
(allowing for large values of MAX_NUM_LAYERS with minimal waste of memory)
3. wrap map->layers access with GET_LAYER macros to ease future developments
(i.e. removal of MAX_NUM_LAYERS)
4. check that layer->map is not null before using it and raise a new
error if it is (MS_NULLPARENTERR)
5. reference counter for mapObj and layerObj
6. modification of mapserver core and swig interface files to do reference
counting for map and layers (incr and decr)
7. created unit tests for python (mapscript/python/tests/cases/refcount.py) to
test the functionality of the previous items. This has not been added to
runtests.py yet

Due to the large extent of the changes I have also made available
items 1,2,3 and item 4 available as two separate patches.

Happy hacking and tell me what you think,
Umberto