fix: only use window scale in children

[Nerixyz]

Bug I introduced in #5794, where the window would call scale(), which would attempt to load scale_ from itself, but that's the variable it wanted to initialize, so now it read garbage and initialized that variable to garbage and then subsequent reads from scale_ would yield that garbage until the window loaded the scale setting.

Not sure if that's the cause of #5831 (I couldn't replicate this¹).

Footnotes

1. Not the exact issue that is. On MSVC, the scale was initialized to a negative value, meaning that calls to setMinimumSize would print an error in Qt. ↩