function log2(v) {
    var r = (v > 0xFFFF ? 1 : 0) << 4;
    v >>>= r;
    var shift = (v > 0xFF ? 1 : 0) << 3;
    v >>>= shift;
    r |= shift;
    shift = (v > 0xF ? 1 : 0) << 2;
    v >>>= shift;
    r |= shift;
    shift = (v > 0x3 ? 1 : 0) << 1;
    v >>>= shift;
    r |= shift;
    return r | (v >> 1);
}
Computes ceil of log base 2
@function log2
@memberof PIXI.utils
@param {number} v - input value
@returns {number} logarithm base 2