Bug fix: preservation of viewport doesnt work

[fstoe]

I recently found out that setting preserveViewport in the osd settings does nothing.
So I made some little changes to the OpenSeadragonViewer Component and to the setcanvas action.

[marlo-longley]

I found this negative logic a bit hard to follow (not from what you wrote, but from what existed before!) -- what do you think about adding comments similar to:

 if (canvasesChanged && viewer.preserveViewport) {
        // Do not reset the zoom
        this.addAllImageSources(false);
      } else {
       // Reset the zoom if the canvas has changed or if there is no viewerConfig
        this.addAllImageSources((canvasesChanged || !viewerConfig));
      }
...

?

Personally the name zoomAfterAdd parameter did not help me understand what was going on. Maybe resetZoomAfterAdd would have been better, or maybe it's just me!

[marlo-longley]

@fstoe I tested this locally and it works well, thank you for submitting.