Include a COPYING inside perf/, test/, util/ to clarify the licensing
conditions beneath the respective directories. This is because cairo
itself (libcairo.so) is LGPL-2.1/MPL-1.1 but that only relates to src/.
The auxiliary source files are under a mix of free licenses and we wish to
be clear just what license applies to each file.

In particular, cairo-trace needs to include the GPL terms and conditions.