seat: start transaction on focus-output

This fixes a delay due to the missing startTransaction() call causing
unapplied pending state.

 river/command/focus_output.zig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/river/command/focus_output.zig b/river/command/focus_output.zig
index b2520ec..1bce2a4 100644
--- a/river/command/focus_output.zig
+++ b/river/command/focus_output.zig
@@ -50,4 +50,5 @@ pub fn focusOutput(
     });
 
     seat.focus(null);
+    root.startTransaction();
 }