[scene-xrender] Implement support for DecorationShadow

The DecorationShadow on XRender is implemented by creating the
required QPixmaps in ::prepareBackend. That way the rendering
infrastructure can be shared with the window based shadows.

BUG: 342758
REVIEW: 122009

scene_xrender.cpp

@@ -1234,8 +1234,28 @@ void SceneXRenderShadow::buildQuads()
 bool SceneXRenderShadow::prepareBackend()
 {
     if (hasDecorationShadow()) {
-        // TODO: implement for XRender
-        return false;
+        const QImage shadowImage = decorationShadowImage();
+        QPainter p;
+        int x = 0;
+        int y = 0;
+        auto drawElement = [this, &x, &y, &p, &shadowImage] (ShadowElements element) {
+            QPixmap pix(elementSize(element));
+            pix.fill(Qt::transparent);
+            p.begin(&pix);
+            p.drawImage(0, 0, shadowImage, x, y, pix.width(), pix.height());
+            p.end();
+            setShadowElement(pix, element);
+            return pix.size();
+        };
+        x += drawElement(ShadowElementTopLeft).width();
+        x += drawElement(ShadowElementTop).width();
+        y += drawElement(ShadowElementTopRight).height();
+        drawElement(ShadowElementRight);
+        x = 0;
+        y += drawElement(ShadowElementLeft).height();
+        x += drawElement(ShadowElementBottomLeft).width();
+        x += drawElement(ShadowElementBottom).width();
+        drawElement(ShadowElementBottomRight).width();
     }
     const uint32_t values[] = {XCB_RENDER_REPEAT_NORMAL};
     for (int i=0; i<ShadowElementsCount; ++i) {

shadow.cpp

@@ -356,4 +356,9 @@ QSize Shadow::elementSize(Shadow::ShadowElements element) const
     }
 }
 
+void Shadow::setShadowElement(const QPixmap &shadow, Shadow::ShadowElements element)
+{
+    m_shadowElements[element] = shadow;
+}
+
 } // namespace

shadow.h

@@ -155,6 +155,7 @@ protected:
     };
     virtual bool prepareBackend() = 0;
     WindowQuadList m_shadowQuads;
+    void setShadowElement(const QPixmap &shadow, ShadowElements element);
 
 private:
     static Shadow *createShadowFromX11(Toplevel *toplevel);