implemented cursors in the timeline

Note: this does not use the cursor SVGs in app/ui/cursors since I was never
really satisfied with them. They're kept for posterity for the time being.

app/render/backend/videorenderworker.cpp

@@ -41,6 +41,7 @@ NodeValueTable VideoRenderWorker::RenderInternal(const NodeDependency& path, con
     emit CompletedFrame(path, job_time, hash, texture);
 
     // If we actually have a texture, download it into the disk cache
+    qDebug() << "Texture was null:" << texture.isNull();
     if (!texture.isNull()) {
       Download(path, hash, texture, frame_cache_->CachePathName(hash));
     }