Add a COSMIC_DISABLE_SYNCOBJ env var to disable syncobj protocol

If any issues with the syncobj protocol come up, or any issues we
suspect may be related to it, an env var may be helpful.

src/backend/kms/mod.rs

@@ -156,6 +156,7 @@ pub fn init_backend(
         }
     }
 
+    if !crate::utils::env::bool_var("COSMIC_DISABLE_SYNCOBJ").unwrap_or(false) {
         let kms = match &mut state.backend {
             BackendData::Kms(kms) => kms,
             _ => unreachable!(),
@@ -172,6 +173,7 @@ pub fn init_backend(
                 }
             }
         }
+    }
 
     Ok(())
 }