Support applying scale outside of animation

[yshui]

Discussed in https://github.com/yshui/picom/discussions/1447

^{Originally posted by dkaszews August 15, 2025} Reading the documentation, I cannot see a way to window's scale outside of an animation. I would like to be able to change window scaling permanently, e.g. to make unfocused windows slightly smaller. I tried to do it using a zoom shader but it unfortunately does not work that well, as it creates a transparent border around the window, since it is still considered full size when calculating things like rounded corners or shadows:

Since there already seems to be an infrastructure for scaling during an animation, I hope this feature is not that hard to implement, though of course it might interact oddly with other pieces of code (such as window size matchers).

[yshui]

I don't think anything more is needed on the rendering side. but a configuration option needs to be added, this option should be supported in rules. and when it changes it needs to trigger the appropriate animation (i think the size trigger is probably the right one).