app: improve splash screen sizing.
Current code was using the dimension of the screen as a max size. That is really too big. 2/3 of the screen size is an acceptable size being both well visible and not overwhelming. Also the current code was cropping, not rescaling, the splash image, which is obviously not an acceptable solution because on a very small displays, we would end up with ununderstandable piece of a bigger image. This new code will allow to ship big size default splash image(s), and display it in an acceptable size on both low and high density displays. We indeed got a feedback from someone with a 4K display who was saying the current dev splash screen was tiny on one's display. Of course, custom splash can still be at any size; but from now on, we will need for the *default upstream splash image(s)* to be of huge dimension in order to show up well everywhere (at least Full HD splash, which is half of a 4K UHD screen). Animated splash images are still not resized though and will show up at their default dimension. This means we cannot ship animated splash screens as a default for the time being (they can still be installed as custom splash).
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