sequenceMode should be true in openseadragon viewer

[nicolasfranck]

When using multiple IIIF tile_sources,

• the prev/next buttons don't work (switching between tile sources)
• openseadragon starts fetching ALL tiles for all tile sources, leading to hundreds of requests in the background
• at startup the viewer goes rapidly through all pages, and ends at the last page

When you change these parameters, it starts working:

app/views/catalog/_openseadragon_default.html.erb:

osd_config_referencestrip = {
    sequenceMode: true
    showReferenceStrip: false
    ...
}

(I set showReferenceStrip to false, because openseadragon shows the refence strip INSIDE the viewer)