Fix T85035: Gpencil render in background render wrong frame

Due a thread priority, the calculation of the current frame is not done before the frames are available. This produces wrong render frames.

The solution is verify the current frame before doing the real render. This adds only a few milliseconds, but it assures the frame is correct.

As the problem is only when render in background, the recalculation of the frame is only necessary when do real render and not a viewport render, so there is no impact in the animation FPS.

source/blender/draw/engines/gpencil/gpencil_engine.c

@@ -618,6 +618,14 @@ void GPENCIL_cache_populate(void *ved, Object *ob)
     bGPdata *gpd = (bGPdata *)ob->data;
     bool do_onion = (!pd->is_render) ? pd->do_onion : (gpd->onion_flag & GP_ONION_GHOST_ALWAYS);
 
+    /* When render in background the active frame could not be properly set due thread priority
+     * better set again. This is not required in viewport. */
+    if (txl->render_depth_tx) {
+      LISTBASE_FOREACH (bGPDlayer *, gpl, &gpd->layers) {
+        gpl->actframe = BKE_gpencil_layer_frame_get(gpl, pd->cfra, GP_GETFRAME_USE_PREV);
+      }
+    }
+
     BKE_gpencil_visible_stroke_iter(is_final_render ? pd->view_layer : NULL,
                                     ob,
                                     gpencil_layer_cache_populate,