homescreens/folio: Improve performance of wallpaper blur, and enable by default

Fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/306

I added a ShaderEffectSource in between so that the blur only samples the wallpaper when necessary

I also switched to FastBlur which seems to be much, much faster than MultiEffect on the PinePhone.

From testing, this made the performance about on-par with no blur at all.

containments/homescreens/folio/foliosettings.cpp

@@ -170,7 +170,7 @@ void FolioSettings::load()
     m_delegateIconSize = m_applet->config().readEntry("delegateIconSize", 48);
     m_showFavouritesBarBackground = m_applet->config().readEntry("showFavoritesBarBackground", true);
     m_pageTransitionEffect = static_cast<PageTransitionEffect>(m_applet->config().readEntry("pageTransitionEffect", (int)SlideTransition));
-    m_showWallpaperBlur = m_applet->config().readEntry("showWallpaperBlur", false);
+    m_showWallpaperBlur = m_applet->config().readEntry("showWallpaperBlur", true);
 
     Q_EMIT homeScreenRowsChanged();
     Q_EMIT homeScreenColumnsChanged();

containments/homescreens/folio/package/contents/ui/main.qml

@@ -5,6 +5,7 @@ import QtQuick
 import QtQuick.Window
 import QtQuick.Layouts
 import QtQuick.Effects
+import Qt5Compat.GraphicalEffects
 
 import org.kde.kirigami 2.20 as Kirigami
 
@@ -35,23 +36,27 @@ ContainmentItem {
         anchors.fill: parent
 
         sourceComponent: Item {
-            // HACK: wallpaper (to enforce same dimensions that blur uses)
-            MultiEffect {
-                blurEnabled: true
-                blur: 0.0
-                blurMax: 0.0
-                autoPaddingEnabled: false
-                source: Plasmoid.wallpaperGraphicsObject
+            id: wallpaper
+            anchors.fill: parent
+
+            // only take samples from wallpaper when we need the blur for performance
+            ShaderEffectSource {
+                id: controlledWallpaperSource
                 anchors.fill: parent
+
+                sourceItem: Plasmoid.wallpaperGraphicsObject
+                live: blur.visible
+                hideSource: false
+                visible: false
             }
 
             // wallpaper blur
-            MultiEffect {
-                blurEnabled: true
-                blur: 1.0
-                blurMax: 50
-                autoPaddingEnabled: false
-                source: Plasmoid.wallpaperGraphicsObject
+            // we attempted to use MultiEffect in the past, but it had very poor performance on the PinePhone
+            FastBlur {
+                id: blur
+                radius: 50
+                cached: true
+                source: controlledWallpaperSource
                 anchors.fill: parent
                 visible: opacity > 0
                 opacity: Math.min(1,