Fix #131235: Grease pencil: Final rendering not working

The `draw::Manager` needs to be manually synced in render.

source/blender/draw/engines/gpencil/gpencil_render.cc

@@ -242,6 +242,8 @@ void GPENCIL_render_to_image(void *ved,
   const DRWContextState *draw_ctx = DRW_context_state_get();
   Depsgraph *depsgraph = draw_ctx->depsgraph;
 
+  DRW_manager_get()->begin_sync();
+
   GPENCIL_render_init(vedata, engine, render_layer, depsgraph, rect);
   GPENCIL_engine_init(vedata);
 
@@ -254,6 +256,8 @@ void GPENCIL_render_to_image(void *ved,
 
   DRW_render_instance_buffer_finish();
 
+  DRW_manager_get()->end_sync();
+
   /* Render the gpencil object and merge the result to the underlying render. */
   GPENCIL_draw_scene(vedata);