demo: fbv

It would be cool to compile [fbv](https://github.com/smokku/fbv) to wasi, and just use framebuffer to output images. I think this would be a good demo to make sure the framebuffer is sound enough for use in existing C codebases.

Start by reviewing the fbv project and the repository's existing WASI framebuffer support, since no source files or tests are named in the issue. Determine the compilation and image-output requirements; done means fbv builds for WASI and uses the framebuffer to display images as a working demo.

Written by the indexing model from the issue text.