Add support for offscreen GL contexts

[jonathanhogg]

It'd be great to be able to create a non-window context in Flitter, for containing scene nodes that will be used only as references and to allow generating images and videos in a non-windowed environment – which would be great for doing functional tests.

[jonathanhogg]

Obvious syntax would be something equivalent to !window, like:

!offscreen id=:foo size=X;Y
    !image ...
    !video ...
    !canvas ...
    !canvas3d
    !shader ...

It almost certainly also makes sense to only create a framebuffer for an !offscreen if it has an id attribute that would see it placed in the references map. Otherwise, there's no way of accessing the output so it might as well not be rendered.

If it does have an id attribute, then it should support the sane range of fragment, vertex, colorbits, linear and composite, though not vsync, title, screen, resizable or fullscreen. !key or !pointer event nodes would not be valid inside one.

It would also be convenient to have an --offscreen command-line option that forces all !window nodes to be interpreted as !offscreen nodes instead.

[jonathanhogg]

So basic offscreen support turned out to be super easy. What will need a bit more thinking about, to avoid a nasty hack, is how to add an --offscreen command-line option.