[scene] Release previous pixmap when a raster buffer is attached

We need to release the previous window pixmap if the new pixmap is
valid. However, it's currently the case only when the client has
attached either an fbo buffer or a wl_buffer. If an internal client
has attached a raster buffer, the previous window pixmap won't be
released.

In order to ensure that we're going to release the previous window
pixmaps no matter what type of buffer has been attached, this change
refactors WindowPixmap to use isValid() to verify that the new
window pixmap is valid.

Differential Revision: https://phabricator.kde.org/D29131