产生一个[-1,1]的随机数

#define    RAND_LIMIT    32767
/// Random number in range [-1,1]
inline float32 RandomFloat()
{
    float32 r = (float32)(std::rand() & (RAND_LIMIT));
    r /= RAND_LIMIT;
    r = 2.0f * r - 1.0f;
    return r;
}