message: add 'kUNKNOWN' image type to frame_buffer.h, for easier format conversion handling
diff: @@ -75,7 +75,7 @@ namespace vision {
  class FrameBuffer {
  public:
  // Colorspace formats.
  - enum class Format { kRGBA, kRGB, kNV12, kNV21, kYV12, kYV21, kGRAY };
  + enum class Format { kRGBA, kRGB, kNV12, kNV21, kYV12, kYV21, kGRAY, kUNKNOWN};
  // Stride information.
  struct Stride {
  