kwineffects: Support setting opacity on OffscreenQuickView

Signed-off-by: Eike Hein <eike.hein@mbition.io>

src/libkwineffects/kwinoffscreenquickview.cpp

@@ -383,6 +383,16 @@ QRect OffscreenQuickView::geometry() const
     return d->m_view->geometry();
 }
 
+void OffscreenQuickView::setOpacity(qreal opacity)
+{
+    d->m_view->setOpacity(opacity);
+}
+
+qreal OffscreenQuickView::opacity() const
+{
+    return d->m_view->opacity();
+}
+
 QQuickItem *OffscreenQuickView::contentItem() const
 {
     return d->m_view->contentItem();

src/libkwineffects/kwinoffscreenquickview.h

@@ -94,6 +94,9 @@ public:
     void setGeometry(const QRect &rect);
     QRect geometry() const;
 
+    void setOpacity(qreal opacity);
+    qreal opacity() const;
+
     /**
      * Render the current scene graph into the FBO.
      * This is typically done automatically when the scene changes

src/scenes/opengl/scene_opengl.cpp

@@ -337,18 +337,28 @@ void SceneOpenGL::paintDesktop(int desktop, int mask, const QRegion &region, Scr
 
 void SceneOpenGL::paintOffscreenQuickView(OffscreenQuickView *w)
 {
-    GLShader *shader = ShaderManager::instance()->pushShader(ShaderTrait::MapTexture);
-    const QRect rect = w->geometry();
-
     GLTexture *t = w->bufferAsTexture();
     if (!t) {
         return;
     }
 
+    ShaderTraits traits = ShaderTrait::MapTexture;
+    const qreal a = w->opacity();
+    if (a != 1.0) {
+        traits |= ShaderTrait::Modulate;
+    }
+
+    GLShader *shader = ShaderManager::instance()->pushShader(traits);
+    const QRect rect = w->geometry();
+
     QMatrix4x4 mvp(renderTargetProjectionMatrix());
     mvp.translate(rect.x(), rect.y());
     shader->setUniform(GLShader::ModelViewProjectionMatrix, mvp);
 
+    if (a != 1.0) {
+        shader->setUniform(GLShader::ModulationConstant, QVector4D(a, a, a, a));
+    }
+
     glEnable(GL_BLEND);
     glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
     t->bind();

src/scenes/qpainter/scene_qpainter.cpp

@@ -130,7 +130,10 @@ void SceneQPainter::paintOffscreenQuickView(OffscreenQuickView *w)
     if (buffer.isNull()) {
         return;
     }
+    painter->save();
+    painter->setOpacity(w->opacity());
     painter->drawImage(w->geometry(), buffer);
+    painter->restore();
 }
 
 Scene::Window *SceneQPainter::createWindow(Toplevel *toplevel)