Looks like we might be using very out-of-date GL APIs; 'fixed-function' instead of shader-based.
https://www.ics.com/blog/fixed-function-modern-opengl-part-1-4

Some work in better-gl

This is looking sort-of-OK, perhaps try it on Windows/macOS and see what happens.

Builds and runs on macOS now.

But is broken again on Ubuntu 20.04.

Fixed on macOS/Ubuntu, doesn't build on Windows; first thing would be an updated wxWidgets in mxe.

This updated wxwidgets dep might need a fair bit of work, but I think it's necessary to request the OpenGL version we need.

mxe git has 3.1.4 of wxWidgets.

Tried rebasing carl-2 of mxe.git onto mxe.cc master in carl-3.

Crashes without an error on Windows.

Kind of working on windows now; main problem was a missing initialisation of glew

Assuming this works on all platforms, one thing to try might be to do the scale and XYZ->RGB in the shader just for the DCP playback case; i.e. check for no crop, image size = container size, no subtitles, no fade, then do the equivalent of PlayerVideo::make_image() in the shader. Would need to avoid PlayerVideo::prepare doing make_image() as it would be a waste of time.

It seems like it might be nice to encode the behaviour of J2KImageProxy and PlayerVideo;
they can return a thing T which they will calculate with some prepare(); you can either
call prepare() in advance and get T quickly or just get T without prepare() and it will
be slower.

Maybe some wrapper for T?

class PlayerVideo
{
    WrappedImage get() {

    }

    void prepare() {
        _wrapped.prepare();
    }
};

class WrappedT
{
    O thing;
    optional<T> result;

    void prepare() {
        // do something to thing to get result
    }

    T get () {
        if (!result) {
            result = prepare();
        }
        return *result;
    }
}