LocalPlayer::accelerateHorizontal: cleanups

LocalPlayer::accelerateHorizontal: cleanups

* Properly use v3f default constructor
* v3f d_wanted = target_speed - m_speed; and d_wanted = target_speed * 0.1f - m_speed * 0.1f; can be factorized to d_wanted = (target_speed - m_speed) * 0.1f; => d_wanted *= 0.1f;