Fix #127292: Cryptomatte passes are missing meta-data

Meta-data are missing on Cryptomatte layers in the GPU compositor, so
they do not get saved using the File Output node. This is due to a use
after free error where a temporary string is used in the meta-data
population logic. This is fixed by assigning the string to a temporary
variable instead.

Thanks to Jorn Visser for finding the cause of the issue.

source/blender/render/intern/compositor.cc

@@ -394,8 +394,9 @@ class Context : public realtime_compositor::Context {
 
     /* We assume the given pass is a Cryptomatte pass and retrieve its layer name. If it wasn't a
      * Cryptomatte pass, the checks below will fail anyways. */
+    const std::string combined_pass_name = std::string(view_layer->name) + "." + pass_name;
     StringRef cryptomatte_layer_name = bke::cryptomatte::BKE_cryptomatte_extract_layer_name(
-        std::string(view_layer->name) + "." + pass_name);
+        combined_pass_name);
 
     struct StampCallbackData {
       std::string cryptomatte_layer_name;