Ability to opt-out of having threaded QImage loading

Since https://codereview.qt-project.org/c/qt/qtbase/+/282779 added feature to draw images with multiple threads has the memory usage (esp. in embedded) increased due to the fact that thread stack usage (allocated/reserved) can be high (default thread stack size on linux is 8MB) and basically increases permanently application memory usage even after it does not load any image (that size is bigger than the defined threshold for doing threaded fills).

On embedded hw where available memory is limited this is bigger problem than the (very insignificant or actually negative) implied performance improvement of using threaded fills.

So having way to disable completely the threaded fills and/or having way to set the level when it's triggered is important so users can control what they prefer more. Less memory consumed of (possible) improvement or loading lot's of large images.

All measurement in the attach file are done with Rpi4 using Qt 6.8.0-RC with patch to disable threading via env variable.