Looking for guidance on windowless operations

[furroy]

My use case, I want to submit an HDR image to a bloom filter, and get back an HDR image I can save to disk. I don't want a window, if I have to have a window, I don't want it visible. The images would also be much higher resolution than the window. I don't want any geometry, cameras, lights, etc, but from what I can tell so far, I suspect I will have to make a simple scene w/ camera and single quad at a minimum. I will already have a thread this is running on, so if it's a single threaded blocking call(s), that's fine. All the bgfx examples seem to require a window and you fire off a render and bgfx does everything behind the scenes. I saw I can setup a callback for saving a screenshot, but this wouldn't be an HDR image would it? I'm using OpenEXR for HDR images, it will need to work on Windows & Mac.