[wayland] Set depth in ShellClient depending on whether the Buffer has alpha

We need to set the depth in order to properly determine whether the
Surface has an alpha channel and whether blending needs to be enabled
for rendering.

For this a new method is introduced in Toplevel to set the depth. If
the depth changed in a way that the Toplevel gained or lost the alpha
channel a signal is emitted which implies that the hasAlpha property of
Toplevel is no longer constant.

shell_client.cpp

@@ -165,6 +165,7 @@ void ShellClient::addDamage(const QRegion &damage)
         }
         setGeometry(QRect(position, m_clientSize));
     }
+    setDepth(m_shellSurface->surface()->buffer()->hasAlphaChannel() ? 32 : 24);
     setReadyForPainting();
     Toplevel::addDamage(damage);
 }

toplevel.cpp

@@ -42,6 +42,7 @@ namespace KWin
 
 Toplevel::Toplevel()
     : m_visual(XCB_NONE)
+    , bit_depth(24)
     , info(NULL)
     , ready_for_painting(true)
     , m_isDamaged(false)
@@ -475,6 +476,18 @@ QByteArray Toplevel::windowRole() const
     return QByteArray(info->windowRole());
 }
 
+void Toplevel::setDepth(int depth)
+{
+    if (bit_depth == depth) {
+        return;
+    }
+    const bool oldAlpha = hasAlpha();
+    bit_depth = depth;
+    if (oldAlpha != hasAlpha()) {
+        emit hasAlphaChanged();
+    }
+}
+
 } // namespace
 
 #include "toplevel.moc"

toplevel.h

@@ -71,7 +71,7 @@ class Toplevel
     : public QObject
 {
     Q_OBJECT
-    Q_PROPERTY(bool alpha READ hasAlpha CONSTANT)
+    Q_PROPERTY(bool alpha READ hasAlpha NOTIFY hasAlphaChanged)
     Q_PROPERTY(qulonglong frameId READ frameId)
     Q_PROPERTY(QRect geometry READ geometry NOTIFY geometryChanged)
     Q_PROPERTY(QRect visibleRect READ visibleRect)
@@ -414,6 +414,10 @@ Q_SIGNALS:
      * @since 5.3
      **/
     void surfaceIdChanged(quint32);
+    /**
+     * @since 5.4
+     **/
+    void hasAlphaChanged();
 
 protected Q_SLOTS:
     /**
@@ -460,6 +464,7 @@ protected:
     friend QDebug& operator<<(QDebug& stream, const Toplevel*);
     void deleteEffectWindow();
     virtual bool shouldUnredirect() const = 0;
+    void setDepth(int depth);
     QRect geom;
     xcb_visualid_t m_visual;
     int bit_depth;