Smooth fade-in/fade-out for SkyImages

[gzotti]

Is your feature request related to a problem? Please describe.
Found while preparing a script: I'd like to show an image insert in the sky. There is ScreenImage and SkyImage.
Only the former can show effects like fade-in or fade-out, or even grow from a spot.
The SkyImage can be loaded and appears/disappears suddenly, which may look ugly under some circumstances.

Describe the solution you'd like
I am investigating options to let a SkyImage smooth in gradually. Unfortunately this is concerning the ill-documented StelSkyLayer/MultilevelJsonBase/StelSkyImageTile family.
It appears that StelSkyImageTile::drawTile() would already apply a fader on first show of an image. However, I never can see this in action. And then once run, it is never fading out. Is this the place to play around with? Or rather StelSkyImageTile::draw() with its currently unused opacity?

Describe alternatives you've considered
ScreenImage. It is OK on a flat sceen, but not in a dome planetarium.

Additional context
None currently.

[github-actions]

Hello @gzotti!

Thank you for suggesting this feature.