Better loader and investigate performance issues

[RFSH]

If you try to open/close the channel panel while image tiles are loading, it will not respond right away and depending on how long the tiles take to load it will be laggy.

You can experience this more often on larger images with larger number of possible zoom. For example,

• Open this image: https://dev.rebuildingakidney.org/chaise/viewer/#2/Gene_Expression:Image/id=14-481P?waterMark=gudmap.org&meterScaleInPixels=3082824
• Click on "Zoom in" button multiple times.
• While the spinner is showing up indicating that images are loading, click on the channel button.
• It will not properly show the panel until the tiles are loaded.

I'm not entirely sure why we're experiencing this and need to investigate more. But I'm assuming it could be because of osd loading/hiding/caching tiles while interacting with the page.

Also as part of this we should make sure the loader is more visible. I added the loader as a debugging tool and didn't want it to be distracting. But now it's way too hidden. One suggesting is showing a banner with loading... text on top of the image.

[RFSH]

Changed the spinner in https://github.com/informatics-isi-edu/openseadragon-viewer/commit/ca99368bd84e340acd779ffe04cb99e1a5fbc02e commit. I still need to look more into the performance issue.

[RFSH]

While working on the channel controls changes, I spend some time looking at the performance issue. Disabling the image color manipulation that we're doing, significantly improves the performance of the app. And that makes sense given that we're going over pixels of the displayed tiles one by one and modify their values.

We might be able to improve this by improving that implementation. There might be some built-in feature in OSD that allows us to do this more efficiently.

[RFSH]

Moved to wishlist.