Make image switching more seamless
I tested on slow hardware within a virtual machine. Here is a video showing how switching between copies of the same image can lead to a completely seamless switching experience:
Especially when animations are disabled, any switching of images is quite jarring in Gwenview. There is always a split second between displaying of images that shows a blank page with a loading indicator. This is unnecessary flickering and makes comparison of similar images more difficult.
This merge request contains two commits that address different parts of this issue.
The first commit delays the showing of the loading indicator. This indicator would instantly show up whenever any loading happens, which is unnecessary and distracting, as it doesn't actually notify the user of anything if the loading is pretty much instantaneous.
The second commit makes the switching of images seamless when animations are disabled. It does so by disabling the updating/repainting of the view container until the loading of the new image has completed(). If loading takes long enough that we would want to show a loading indicator, the seamless experience is given up on and we do show a loading indicator instead.