Replace all hardcoded 1e30(f) by BT_LARGE_FLOAT, defined in btScalar.h as 1e18(f) so that its square still fits in FLT_MAX

Thanks to Ole K. for reporting! http://code.google.com/p/bullet/issues/detail?id=206

Demos/VehicleDemo/VehicleDemo.cpp

@@ -66,7 +66,7 @@ float	steeringIncrement = 0.04f;
 float	steeringClamp = 0.3f;
 float	wheelRadius = 0.5f;
 float	wheelWidth = 0.4f;
-float	wheelFriction = 1000;//1e30f;
+float	wheelFriction = 1000;//BT_LARGE_FLOAT;
 float	suspensionStiffness = 20.f;
 float	suspensionDamping = 2.3f;
 float	suspensionCompression = 4.4f;