This creates a new independent executable that exercise the entire llimage libray, loading and saving all supported image file formats, gathering perf data and outputting basic image info.

The use of the application itself is documented in the wiki: https://wiki.secondlife.com/wiki/Llimage_libtest

From a code standpoint, the main things done are:
- Add a new test application in indra/integration_tests/llimage_libtest/
- Move some perf amalyzing code from indra/newview/llfasttimerview.cpp to indra/llcommon/llmetricperformancetester so that it can be used outside the newview context (as it should...)
- Small cleanup in llimage.h

So far, testing has been done on Mac only. Tests on Windows and Linux are planned and will likely result in cmake tweaks here and there. We're not planning however to alter dramatically this code further at this point.