scene: use fractionalGeometry() as input of RenderViewport renderRect

Fixes mismatches between renderTarget.size() and deivce geometry calculated using viewport.renderRect() * viewport.scale() .

BUG: 475325

src/scene/workspacescene.cpp

@@ -372,7 +372,7 @@ void WorkspaceScene::postPaint()
 void WorkspaceScene::paint(const RenderTarget &renderTarget, const QRegion &region)
 {
     Output *output = kwinApp()->operationMode() == Application::OperationMode::OperationModeX11 ? nullptr : painted_screen;
-    RenderViewport viewport(output ? output->geometry() : workspace()->geometry(), output ? output->scale() : 1, renderTarget);
+    RenderViewport viewport(output ? output->fractionalGeometry() : workspace()->geometry(), output ? output->scale() : 1, renderTarget);
 
     m_renderer->beginFrame(renderTarget, viewport);