Initialize member variable QSGAnimationDriver::m_lag.

Fixes purify warning:
[W] UMR: Uninitialized memory read in QSGAnimationDriver::advance(void) {1 occurrence}
QSGAnimationDriver::advance(void) [qtdeclarative\src\quick\scenegraph\qsgcontext.cpp:221]

Task-number: QTBUG-42213
Change-Id: Ide35830c60c5935644747da5cfbad1cdbd357232
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>