Properly set m_size when updating SurfacePixmap{Wayland,Internal}

Otherwise the size() getter in the base class outputs an invalid size
and we can't really use it.

src/surfaceitem_internal.cpp

@@ -9,6 +9,8 @@
 #include "internalwindow.h"
 #include "scene.h"
 
+#include <QOpenGLFramebufferObject>
+
 namespace KWin
 {
 
@@ -71,9 +73,11 @@ void SurfacePixmapInternal::update()
 
     if (window->internalFramebufferObject()) {
         m_fbo = window->internalFramebufferObject();
+        m_size = m_fbo->size();
         m_hasAlphaChannel = true;
     } else if (!window->internalImageObject().isNull()) {
         m_rasterBuffer = window->internalImageObject();
+        m_size = m_rasterBuffer.size();
         m_hasAlphaChannel = m_rasterBuffer.hasAlphaChannel();
     }
 }

src/surfaceitem_wayland.cpp

@@ -193,6 +193,7 @@ void SurfacePixmapWayland::setBuffer(KWaylandServer::ClientBuffer *buffer)
     if (m_buffer) {
         m_buffer->ref();
         m_hasAlphaChannel = m_buffer->hasAlphaChannel();
+        m_size = m_buffer->size();
     }
 }