Thumbnail rendering glitch

[timvandermeij]

After PR #6299 there is a (somewhat minor) visual glitch in the thumbnail viewer when a thumbnail is being rendered. See #6441 for a screenshot. I'm tentatively assigning this to @Snuffleupagus as he appears to have an idea on how to solve this.

I'm also adding https://stackoverflow.com/questions/14396356/force-firefox-to-render-empty-image-within-dimensions-specified-by-css for more documentation on how to possibly solve this. From that page it appears that this issue is Firefox-only.

[Snuffleupagus]

Conceptually, I'd actually prefer to re-factor this a bit.
My preferred solution would be to (again) render thumbnails on a visible canvas, and once the rendering is done convert to an image and then swap the elements; please see a WIP patch here.
That seems to me to be a slightly more robust solution over all, but unfortunately it doesn't seem to be working perfectly. When testing this there were intermittent flickering (at least in Firefox) when doing replaceChild, which didn't look particularly good.

Since we're kind of short on time here, before the next Firefox release, I choose to submit a much simpler fix which should be an improvement for now. We can then revisit this later on, if we want to.

[timvandermeij]

I actually think this current patch does the work very well. We can indeed always refactor this later on if we wish, but swapping elements does seem a bit tricky to me too to get right.