Add a multi-window example

[aclysma]

This library would ideally handle multiple windows easily. Some structural changes may be necessary to handle this, and an example should be added to demonstrate how to do it.

[kasper0406]

I tried creating a multi-window application.
I managed to do it after making the following change #105

[aclysma]

For what it's worth, ideally a multi-window solution would only initialize the API once. Initializing the API per-window prevents sharing resources between the windows (i.e. upload a texture to the GPU once and render it on both windows.) And any initialization skia does will need to be performed once per window (for example, building vulkan graphics pipelines.

That said, this is likely the simplest way to have multiple windows as long as you can live with this limitation.

[ctrlcctrlv]

We're not sure we can handle that limitation in MFEK, chiefly because it'd require multiple instantiation of Dear ImGui. Multiple windows has been something I've been wanting to explore for a while. @kasper0406, feel free to email me—funding is available for work in this area, if you write patches freelance.