code: function BmpDecoder(buffer,is_with_alpha) {
    this.pos = 0;
    this.buffer = buffer;
    this.is_with_alpha = !!is_with_alpha;
    this.bottom_up = true;
    this.flag = this.buffer.toString("utf-8", 0, this.pos += 2);
    if (this.flag != "BM") throw new Error("Invalid BMP File");
    this.parseHeader();
    this.parseRGBA();
  }
docs: @author shaozilee
  
  Bmp format decoder,support 1bit 4bit 8bit 24bit bmp