This define is quite useful for debugging scene graph internals, but
it is disabled by default to conserve memory.

For clarity, I renamed the define to QSG_RUNTIME_DESCRIPTION.

Change-Id: Ie5ff44d67af38adc65d0d09255d8533dc7a33bff
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>