run the PhysicsServerExample 'stepSimulation' smoother

fix a warning in ThirdParty/stb_image
fix an single/double precision issue in btPolyhedralContactClipping.cpp

src/BulletCollision/NarrowPhaseCollision/btPolyhedralContactClipping.cpp

@@ -116,7 +116,7 @@ static int gActualSATPairTests=0;
 
 inline bool IsAlmostZero(const btVector3& v)
 {
-	if(fabsf(v.x())>1e-6 || fabsf(v.y())>1e-6 || fabsf(v.z())>1e-6)	return false;
+	if(btFabs(v.x())>1e-6 || btFabs(v.y())>1e-6 || btFabs(v.z())>1e-6)	return false;
 	return true;
 }