At least the framebuffer backend does not have support for an overlay
cursor. Thus the cursor needs to be rendered by the scene. This change
allows a backend to set that it needs a software cursor which triggers
tracking in the AbstractBackend. A repaint for the old cursor region is
triggered whenever the cursor pos changes.
So far only the QPainter/framebuffer scene is adjusted to render the
software cursor. This is done after rendering a frame with the up to
date cursor position.
There is one problem, though: the KWin internal cursors don't work
as we need to get it from the theme. Using wayland-cursor doesn't help
as it gives us a (client) wl_buffer* and we cannot read the memory back.
The AbstractBackend provides a virtual method to create the Screens.
A concrete backend can implement this method to create the backend
specific Screens instance.
This removes the casting logic from Screens::create.
The AbstractBackend registers itself in the WaylandServer allowing
external users to easily get to the backend and not needing to test
manually which backend is used.
