code: function BmpDecoder(buffer, is_with_alpha) {
    this.pos = 0;
    this.buffer = buffer;
    this.datav = new DataView(buffer.buffer);
    this.is_with_alpha = !!is_with_alpha;
    this.bottom_up = true;
    this.flag = String.fromCharCode(this.buffer[0]) + String.fromCharCode(this.buffer[1]);
    this.pos += 2;
    if (["BM", "BA", "CI", "CP", "IC", "PT"].indexOf(this.flag) === -1) throw new Error("Invalid BMP File");
    this.parseHeader();
    this.parseBGR();
  }
docs: @author shaozilee
  
  Bmp format decoder,support 1bit 4bit 8bit 24bit bmp