libkwineffects: Make Texture export mode default in OffscreenQuickView

Mainly, only aurorae decoration theme needs image export mode. All other
usecases use texture export mode.

src/libkwineffects/kwinoffscreenquickview.cpp

@@ -74,7 +74,7 @@ public:
 };
 
 OffscreenQuickView::OffscreenQuickView(QObject *parent)
-    : OffscreenQuickView(effects ? ExportMode::Texture : ExportMode::Image, parent)
+    : OffscreenQuickView(ExportMode::Texture, parent)
 {
 }
 

src/libkwineffects/kwinoffscreenquickview.h

@@ -65,7 +65,7 @@ public:
     /**
      * Construct a new KWinQuickView explicitly stating an export mode
      */
-    explicit OffscreenQuickView(ExportMode exportMode, QObject *parent = nullptr);
+    explicit OffscreenQuickView(ExportMode exportMode = ExportMode::Texture, QObject *parent = nullptr);
 
     /**
      * Note that this may change the current GL Context