code: public void update(int width, int height, byte[] grayscaleData)
      {
          NativeBitmapImage.updateFromMemory(getNative(), width, height, grayscaleData);
      }
docs: Copy new grayscale data to the GPU texture. This one is also safe even
  in a non-GL thread. An updateGPU request on a non-GL thread will
  be forwarded to the GL thread and be executed before main rendering happens.
  
  Be aware that updating a texture will affect any and all
  {@linkplain GVRMaterial materials} and/or post effects that use the texture!
  @param width     width of grayscale image
  @param height    height of grayscale image
  @param grayscaleData  A byte array containing grayscale data
  
  @since 1.6.3