Expand QSGImageParticle test

Now tests all performance levels.

Also fixes an emitter timing edge-case which might be destabilizing the
tests.

Change-Id: I5e2f133dc4b96dd05d49e7e983ae630e379fa66b
Reviewed-on: http://codereview.qt-project.org/6313


Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>