Decouple window pixmaps from scene windows

One of the scene redesign goals is to make wayland surface items
re-usable. So we have the same rendering path for drag-and-drop icons,
software cursors, and window surfaces.

The biggest issue at the moment is that window pixmaps are tightly
coupled with scene windows.

This change de-couples window pixmaps from scene windows. In order to
achieve that, some architecture changes were made.

The WindowPixmap class was replaced with the SurfacePixmap class. A
surface pixmap is created by a surface item.

Under the hood, a SurfacePixmap will create a PlatformSurfaceTexture
object, which contains all the information necessary for the renderer.

The SceneOpenGLTexture class was removed. However, the GLX and the EGL
on X11 backends still mess with GLTexture's internals.