Support Alpha channel in internal RT when HDR enabled

[zhuxudong]

We use the R11G11B10_UFloat format when WebGL2 and HDR are enabled, which disables the alpha channel. Perhaps we can add a switch to use R16G16B16A16, thereby enabling the alpha channel.

 _getInternalColorTextureFormat(): TextureFormat {
    return this._enableHDR
      ? this.engine._hardwareRenderer.isWebGL2
        ? TextureFormat.R11G11B10_UFloat
        : TextureFormat.R16G16B16A16
      : TextureFormat.R8G8B8A8;
  }