Performance backlog

Issues with the performance tests itself and things that needs to be improved.

Description	Commit
http_server is currently sending every response with "Content-Length". This will make only one path in QHttpNetworkReply for body paring be executed. The http_server must send content with transfer-encoding "chunked" and without the "Content-Length".
Create paint traces from rendering images. This can be useful for Oslo developers to figure out which areas of painting to optimize.
loading_standalone was created to reproduce a loading issue with Wikipedia content. Currently it is not able to reproduce the exact same set of problems. The test should be changed to start new downloads from within didReceiveResponse... like in the real case.
tst_painting is currently using QApplication::syncX in the performance run to make sure that everything has been synced. The problem with this paint benchmark is that on accelerated hardware it is not clear when the image has been painted. The QApplication::syncX will make the test a bit artificial in one way.
memusage is not working on glibc/i686... this is making us blind on memory usage...
Benchmarks should generate more stable timing results.
Change cycling, loading and scrolling to not use the _data and QFETCH but cycle through all sites in the QBENCHMARK and drop caches at the end of one iteration. This should be done because users will navigate from page to page instead of reloading everything and then going to the next one. And with the current QBENCHMARK we should see less variations when cycling through the whole set. Also maybe use an improved QBENCHMARK to log individual parts of the test...	​a80b4