Fix T65614: Sequencer render single layer only

Was noticeable when sequencer uses scene with a compositor.

The way it was using render API was forcing a single render layer.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5095

source/blender/blenkernel/intern/sequencer.c

@@ -3578,7 +3578,8 @@ static ImBuf *seq_render_scene_strip(const SeqRenderData *context,
         re = RE_NewSceneRender(scene);
       }
 
-      RE_RenderFrame(re, context->bmain, scene, view_layer, camera, frame, false);
+      RE_RenderFrame(
+          re, context->bmain, scene, have_comp ? NULL : view_layer, camera, frame, false);
 
       /* restore previous state after it was toggled on & off by RE_RenderFrame */
       G.is_rendering = is_rendering;