feat(video_tool): add next/prev frame command, support prefetching
feat(video_tool): add next/prev frame command, support prefetching

seung-lab/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer