Even if SIXTYFPS_BACKEND_GL is set to OFF, we would default to enabling
X11, which translates to enabling the x11 feature, which in turn
implicitly activates the GL backend. In addition, the interpreter Rust
API crate also enables the GL backend in its defeaults.
In order to make it possible to turn off the GL backend, three changes are necessary:
* The default backend no more selects a default :-). Instead the Rust
API crate, Rust interpreter API crate and the CMake project define
the same features and defaults.
* The C++ crate depends on the interpreter without its defaults (which enable GL)
* In CMake the X11 and Wayland features become dependent options, that
are only show in the CMake configure UI if the GL backend is enabled.
This way a GL-disabled build won't also pass --features x11.
For the Rust API crate we depend on the GL and the Qt feature on the default
backend, but with Corrosion we can't select features yet. So at least
include the GL backend by default.
Currently the Qt backend still redirect everything to the GL backend,
but the goal is to use QPainter and QWindow
This also adds a "default" backend, whose goal is to select the proper
backend at compile time
