Add support for falling back to SIXTYFPS_BACKEND

If `SLINT_BACKEND` isn't set but `SIXTYFPS_BACKEND` is.

Author: tronical

internal/backends/selector/lib.rs

@@ -20,7 +20,13 @@ cfg_if::cfg_if! {
         ))] {
         pub fn backend() -> &'static dyn i_slint_core::backend::Backend {
             i_slint_core::backend::instance_or_init(|| {
-                let backend_config = std::env::var("SLINT_BACKEND").unwrap_or_default();
+                let backend_config = std::env::var("SLINT_BACKEND").or_else(|_| {
+                    let legacy_fallback = std::env::var("SIXTYFPS_BACKEND");
+                    if legacy_fallback.is_ok() {
+                        eprintln!("Using `SIXTYFPS_BACKEND` environment variable for dynamic backend selection. This is deprecated, use `SLINT_BACKEND` instead.")
+                    }
+                    legacy_fallback
+                }).unwrap_or_default();
 
                 #[cfg(feature = "i-slint-backend-qt")]
                 if backend_config == "Qt" {