[scene] Discard pixmaps on buffer size change

The surface size is a logical size, which renders it unsuitable for deciding whether the window pixmap needs to be discarded.

We need to discard window pixmaps when the buffer size changes. That has a drawback though, discarding textures is kind of an overkill with linux-dmabuf. However, fixing that would involve changes in kwayland server that are far from being trivial.