The default backend does not select any features or backends by default
on the Cargo.toml feature level. And with this change it also doesn't do
that anymore based on the target platform.
That means if the rust api crate is built with default-features = false,
no backend will be selected and the user has to take care of backend
initialization through other means.
The feature defaults, generally, are in the rust API crate, the
interpreter crate and C++ CMakeLists.txt. The latter triggers a cargo
build of sixtyfps-cpp with --no-default-features. However the cpp_test
job in the CI doesn't build sixtyfps-cpp with CMake but instead calls
cargo directly. Therefore the defaults are now also coded in the
sixtyfps-cpp Cargo.toml, to ensure that *a* rendering backend is chosen
(since the cfg.target bit is gone).
This allows creating multiple windows for example, and it will allow for
showing windows in those tests that require a mapped window.
As a bonus, the run() function on generated components is not consuming
anymore.
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
