For many, Houston was simply another show on the 1965 North American tour.     However, in many ways it was both a microcosm of that whirlwind month, and a turning point in their touring career.     Years later, George Harrison would recall Houston (and Dallas the prior year) as one of the more frightening bits of "the mania".    Likely courtesy of the rival "Top 40" station, the expected crowd swelled by thousands of extra kids, who quickly overwhelmed an underprepared police force, and actually surrounded the airplane.     The Beatles and their entourage had to be removed by a food catering truck, and Brian was to suffer a serious injury to his back courtesy of the unorthodox method of egress.