[Viewer3D] MediaLoader: Bind fixedPointSize for the SfMLoader

The `fixedPointSize` parameter was used to determine whether the
point size was fixed or programmable from the QML (using a `PointSize`
render state). With Qt6, this render state is not correctly handled by
the RHI and we need to set directly within the shader whether the point
size is fixed or not, hence the binding.

meshroom/ui/qml/Viewer3D/MediaLoader.qml

@@ -108,6 +108,7 @@ import Utils 1.0
             Component.onCompleted: {
                 var obj = Viewer3DSettings.sfmDataLoaderComp.createObject(sfmDataLoaderEntity, {
                                                "source": source,
+                                               "fixedPointSize": Qt.binding(function() { return Viewer3DSettings.fixedPointSize }),
                                                "pointSize": Qt.binding(function() { return 0.01 * Viewer3DSettings.pointSize }),
                                                "locatorScale": Qt.binding(function() { return Viewer3DSettings.cameraScale }),
                                                "cameraPickingEnabled": Qt.binding(function() { return root.enabled && root.cameraPickingEnabled }),