qt: delay requesting the interface window backdrop blur effect only on wayland

In b1e11641, we started to delay requesting the blur effect for the interface window. This was only because of Wayland, as without delaying the blur effect could not be activated.

Even though this problem only occurred with Wayland (I'm not sure if it is a KWin Wayland thing, or a KWindowSystem Wayland thing), the code for delaying requesting the blur effect was not made conditional to simplify things as it is generally fine to have the effect as soon as the window gets activated.

With 874925c0, we disabled using redirection surface on Windows when Direct Composition is used. This makes the window completely transparent.

In early 2025, Chromium started doing the same [1]. However, they mention that it is better to have the blur effect before the window is ready:

Since the browser window appears before the GPU process is able to draw content into it, the acrylic effect gives the user feedback that a window is present and content is coming. Without the acrylic effect a transparent window will appear with a 1 pixel border that eats all mouse clicks; not a good user experience. Further, the acylic effect will appear in uncovered regions of the window when the user resizes the window.

Since we have the same "problem", I propose delaying requesting the blur effect only when necessary, i.e. on Wayland. I have previously checked that on X11 and Windows this was (not delaying) working fine.

This also makes us aligned with the behavior of Chrome browser, along with other applications that are going to follow it.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/6143173

Request review @chub.