qt: flickable_scroll_handler: fix uninitialized variables

setScaleFactor(1.0) was called from the constructor, but then it would
call qFuzzyCompare() with the current scale factor which was not
initialized.

    ==2600289== Thread 10 vlc-qt:
    ==2600289== Conditional jump or move depends on uninitialised value(s)
    ==2600289==    at 0xE8E56F1: qFuzzyCompare (qglobal.h:904)
    ==2600289==    by 0xE8E56F1: ??? (flickable_scroll_handler.cpp:253)
    ==2600289==    by 0xE8E67EB: ??? (flickable_scroll_handler.cpp:36)
    ==2600289==    by 0xE1F5B76: QQmlElement (qqmlprivate.h:139)
    ==2600289==    by 0xE1F5B76: ??? (qqmlprivate.h:166)
    ==2600289==    by 0x104BE4AA: QQmlType::create(QObject**, void**, unsigned long) const (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x10502AE2: QQmlObjectCreator::createInstance(int, QObject*, bool) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x105030AE: QQmlObjectCreator::create(int, QObject*, QQmlInstantiationInterrupt*, int) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x10502A04: QQmlObjectCreator::createInstance(int, QObject*, bool) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x10503B4C: QQmlObjectCreator::setPropertyBinding(QQmlPropertyData const*, QV4::CompiledData::Binding const*) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x105050BF: QQmlObjectCreator::setupBindings(bool) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x10509185: QQmlObjectCreator::populateInstance(int, QObject*, QObject*, QQmlPropertyData const*) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x105026B5: QQmlObjectCreator::createInstance(int, QObject*, bool) (in /usr/lib/libQt5Qml.so.5.15.12)
    ==2600289==    by 0x105030AE: QQmlObjectCreator::create(int, QObject*, QQmlInstantiationInterrupt*, int) (in /usr/lib/libQt5Qml.so.5.15.12)