effects/{blur,contrast}: Move to the front in the effect chain pos

Backdrop effects such as blur and background contrast have to be in the
front of the effect chain because they need to work with background
before the window is painted on top.

Hopefully it's going to fix panel popup blinking visual artifacts.

src/effects/backgroundcontrast/contrast.h

@@ -42,7 +42,7 @@ public:
 
     int requestedEffectChainPosition() const override
     {
-        return 76;
+        return 21;
     }
 
     bool eventFilter(QObject *watched, QEvent *event) override;

src/effects/blur/blur.h

@@ -48,7 +48,7 @@ public:
 
     int requestedEffectChainPosition() const override
     {
-        return 75;
+        return 20;
     }
 
     bool eventFilter(QObject *watched, QEvent *event) override;