Watermark MP4 Video using opengl es on android

I would like to be able to open an mp4 video, watermark it by adding it a png image on top of the video and then save it back to the local storage using android native libraries and opengl es shaders. The videos are 1080p 60fps videos, that are loaded from local storage, then use the opengl es library to overlay a watermark on the video and then save it using media encoder back to mp4 format with the same quality and audio.
The code needs to be neat, commented and easily understood so we can edit and change in case we want to.