function readRle(reader, count, bitWidth, output, seen) {
  const width = bitWidth + 7 >> 3
  let value = 0
  for (let i = 0; i < width; i++) {
    value |= reader.view.getUint8(reader.offset++) << (i << 3)
  }
  // assert(value < 1 << bitWidth)

  // repeat value count times
  for (let i = 0; i < count; i++) {
    output[seen + i] = value
  }
}
Run-length encoding: read value with bitWidth and repeat it count times.

@param {DataReader} reader
@param {number} count
@param {number} bitWidth
@param {DecodedArray} output
@param {number} seen