We want to be able to put existing functionality behind a feature flag while keeping
the semver compatibility.
This is only possible if that new feature flag is enabled by default, but this is not
working if the users have done `default-features = false` in their Cargo.toml.
So we add new `compat-x-y-z` feature that is mandatory to have and which is
enforced with a `compile_error!`

Now, users that whishes to not have the default features must enable it explicitly.
Say we want only x11 but not qt and wayland, the user will do
```toml
sixtyfps = { version = "0.2", default-features = false, features = ["x11", "compat-0-2-0"] }
```

Now, imagine that in the version 0.2.3, we put the SVG support behind a feature flag.
we will do this in out Cargo.toml:

```toml
[features]
default = ["compat-0-2-0", "x11", "wayland"]
compat-0-2-0 = ["compat-0-2-3", "svg"]
compat-0-2-3 = []

svg = [...]
...
```

That way, the svg feature will be enabled by default for all the users who used previous version
of SixtyFPS, and people that want to disable "svg" can just change from compat-0-2-0 to
compat-0-2-3 in their Cargo.toml