Engine is now aware of being reloaded

When game was resumed, Engine::onUpdate(pNanosecondsElapsed)
was called with unusually big value of pNanosecondsElapsed.
This caused Box2D simulation instability or undesired behavior of particle systems.

This patch fixes this issue by adding lag awarness to the Engine.
Vertex, Texture and Font managers will return true from theirs update* method
if feeling guilty of unusually long execution time (due to reloading).