Fix centering of inner circle in PhotoVideoControl

The inner circle in the PhotoVideoControl widget was not always
perfectly centered due to a bug in Qt's anchors.centerIn property
(QTBUG-95224) which causes it to return integer positions instead of
subpixel values.

Implemented a workaround by manually calculating the center position,
ensuring exact centering regardless of parent and child dimensions and
DPI scaling.

This fix should be reverted once QGC moves to Qt 6.8.1 or higher, where
the underlying Qt bug has been resolved.

Related Qt bug: https://bugreports.qt.io/browse/QTBUG-95224

Qt fix commit:
qt/qtdeclarative@fd23a22

src/FlightMap/Widgets/PhotoVideoControl.qml

@@ -165,7 +165,11 @@ Rectangle {
                     border.width:       3
 
                     Rectangle {
-                        anchors.centerIn:   parent
+                        // anchors.centerIn snaps to integer coordinates before
+                        // Qt v6.8.1 (QTBUG-95224), so we need to calculate
+                        // manually for exact subpixel centering
+                        x: (parent.width - width) / 2
+                        y: (parent.height - height) / 2
                         width:              parent.width * (_isShootingInCurrentMode ? 0.5 : 0.75)
                         height:             width
                         radius:             _isShootingInCurrentMode ? 0 : width * 0.5