Minor change.
Merge remote-tracking branch 'nicolasgramlich/GLES2-AnchorCenter' into GLES2-AnchorCenter
Engine is now aware of being reloaded

When game is resumed Engine::onUpdate() was called with unusually big
pNanosecondsElapsed. This caused Box2D simulation instability or strange
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).