gallium: hook up reference count debugging code

gallium: hook up reference count debugging code

This commit adds the ability to produce a log file containing all
reference count changes, and object creation/destruction, on Gallium
objects.

The data allows to answer these crucial questions:
1. This app is exhausting all my memory due to a resource leak: where
   is the bug?
2. Which resources is this app using at a given moment? Which parts of
   the code created them?
3. What kinds of resources does this app use?
4. How fast does this app create and destroy resources? Which parts of
   the code create resources fast?

The output is compatible with the one produced by the similar facility
in Mozilla Firefox, allowing to use Mozilla's tools to analyze the data.

To get the log file:
export GALLIUM_REFCNT_LOG=<file>

To get function names and source lines in the log file:
tools/addr2line.sh <file>

To process the log file, see:
http://www.mozilla.org/performance/refcnt-balancer.html