renderer: if texture is null, don't allocate a frame unless exporting

app/render/backend/renderworker.cpp

@@ -126,7 +126,7 @@ void RenderWorker::RenderFrame(RenderTicketPtr ticket, ViewerOutput* viewer, con
   FramePtr frame = Frame::Create();
   frame->set_timestamp(time);
 
-  if (video_force_download_resolution_ || texture.isNull()) {
+  if (video_force_download_resolution_) {
     // If we're setting the resolution ourselves or we're zeroing it out, allocate the frame now
     frame->set_video_params(VideoParams(video_params_.width(),
                                         video_params_.height(),

app/widget/viewer/viewerdisplay.cpp

@@ -99,7 +99,11 @@ void ViewerDisplayWidget::SetSignalCursorColorEnabled(bool e)
 
 void ViewerDisplayWidget::SetImage(FramePtr in_buffer)
 {
-  last_loaded_buffer_ = in_buffer;
+  if (in_buffer && in_buffer->is_allocated()) {
+    last_loaded_buffer_ = in_buffer;
+  } else {
+    last_loaded_buffer_ = nullptr;
+  }
 
   if (last_loaded_buffer_) {
     makeCurrent();