kwin: reverting some parts of 73cd2c6fa67e2653a8d9a5e17f0140381d998c19

Might results to some asserts in the debug build and does not play well with other effects.

effects/taskbarthumbnail/taskbarthumbnail.cpp

@@ -57,6 +57,9 @@ TaskbarThumbnailEffect::~TaskbarThumbnailEffect()
 
 void TaskbarThumbnailEffect::prePaintScreen(ScreenPrePaintData& data, int time)
 {
+    if (thumbnails.count() > 0) {
+        data.mask |= PAINT_SCREEN_WITH_TRANSFORMED_WINDOWS_WITHOUT_FULL_REPAINTS;
+    }
     effects->prePaintScreen(data, time);
 }