压缩时设置 focusAlpha 参数未传入

[ghost]

在builder初始化时未把 focusAlpha 参数赋值：

 private Luban(Builder builder) {
    this.mTargetDir = builder.mTargetDir;
    this.mRenameListener = builder.mRenameListener;
    this.mStreamProviders = builder.mStreamProviders;
    this.mCompressListener = builder.mCompressListener;
    this.mLeastCompressSize = builder.mLeastCompressSize;
    this.mCompressionPredicate = builder.mCompressionPredicate;
    mHandler = new Handler(Looper.getMainLooper(), this);
  }

能否加入 如果是Png图片，自动设置focusAlpha参数为true，其它设置为false;

[Curzibn]

保留alpha通道的情况下压缩的效率会很低下，只有在业务要求愿意牺牲压缩时间效率的情况下保留alpha通道再去设置focusAlpha参数。一般的业务都不会对透明通道过多要求的。

[cjcj125125]

png压缩后alpha通道变黑色了，有办法把改为其他颜色吗