qt: respect `qt-backdrop-blur` setting also intra-window

I don't see a reason why qt-backdrop-blur should not be respected for intra-window backdrop blur effect (Frosted Glass Effect).

The said setting was first added to disable inter-window backdrop blur, which is currently supported with the following circumstances:

• KWin (Wayland and X11) and KWindowSystem is available.
• Windows 11 22H2 (native acrylic effect DWMSBT_TRANSIENTWINDOW).
• Windows 10/11 simulated acrylic effect (compositor_ dcomp_acrylicsurface.cpp) when Direct Composition is used (--qt-compositor=dcomp).

We also have our own backdrop blur effect, currently used for the mini player and the indexing progress indicator.

I propose that if the user explicitly opt-outs from backdrop blur by unsetting qt-backdrop-blur (it is set by default), the application shall respect that for intra-window backdrop blur effect.

Request review @chub.

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