PhotoVideoControl.qml: fix Slider setting some facts to minimum:

Sliders with a non 0 minimum value have this
issue where the default minimum is 0, so if
setting a new minimum value changes, and thus
we have an onValueChanged, setting the value
of the slider to minimum even before
onCompleted, so if they need to be initialized
to something it doesn't work.

This commit fixes it setting a bool variable
so onValueChanged doesn't take effect until
initialized

src/FlightMap/Widgets/PhotoVideoControl.qml

@@ -467,8 +467,19 @@ Rectangle {
                                 stepSize:                   parent._fact.increment
                                 visible:                    parent._isSlider
                                 updateValueWhileDragging:   false
-                                onValueChanged:             parent._fact.value = value
-                                Component.onCompleted:      value = parent._fact.value
+                                property bool initialized:  false
+
+                                onValueChanged: {
+                                    if (!initialized) {
+                                        return
+                                    }
+                                    parent._fact.value = value
+                                }
+
+                                Component.onCompleted: {
+                                    value = parent._fact.value
+                                    initialized = true
+                                }
                             }
                             QGCSwitch {
                                 checked:        parent._fact ? parent._fact.value : false