code: func (b Bitmap32) PopCount() int {
  	// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
  	b -= (b >> 1) & 0x55555555
  	b = (b>>2)&0x33333333 + b&0x33333333
  	b += b >> 4
  	b &= 0x0f0f0f0f
  	b *= 0x01010101
  	return int(byte(b >> 24))
  }
docs: // PopCount returns the amount of bits set to 1 in the Bitmap32