Fix Qt warnings when rect is invalid, e.g. QRect(0,2111 3840x0)

Summary:
    kwin(28512) QPainter::begin|QPainter::QPainter|KWin::Decoration::Renderer::renderToImage QPainter::begin: Paint device returned engine == 0, type: 3
    kwin(28512) QPainter::setRenderHints|QPainter::setRenderHint|KWin::Decoration::Renderer::renderToImage QPainter::setRenderHint: Painter must be active to set rendering hints
    kwin(28512) QPainter::setWindow|KWin::Decoration::Renderer::renderToImage|?KWinX11Platform.so?  QPainter::setWindow: Painter not active
    kwin(28512) QPainter::setClipRect|KWin::Decoration::Renderer::renderToImage|?KWinX11Platform.so?  QPainter::setClipRect: Painter not active

Test Plan: seems to happen on startup, at least (when restarting kwin)

Reviewers: graesslin, zzag

Reviewed By: zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D9014