OpenGL 3.3 support

[cloudhead]

To turn on the opengl backend, build with cargo build --release --features compatibility

[neauoire]

The current version gives me:

[2020-01-06T14:20:13Z INFO  rx] framebuffer size: 1280x720
[2020-01-06T14:20:13Z INFO  rx] hidpi factor: 2
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[cloudhead]

@neauoire did you try #42 (comment) ?

This output suggests you aren’t running with —feature compatibility

[cloudhead]

You can use ‘cargo run’ directly with those flags to test things out.

[neauoire]

I think I'm doing this right, am I missing a step?

[cloudhead]

@neauoire ah yes it's a bit confusing but cargo install doesn't care what features you used to build (it'll run build in the background), try cargo install --force --locked --path . --features compatibility. If you're just testing things out you can also use cargo run --release --features compatibility.