Port software cursor to RenderLayer

Software cursor has always been a major source of problems. Hopefully,
porting it to RenderLayer will help us with that.

Note that the cursor layer is currently visible only when using software
cursor, however it will be changed once the Compositor can allocate
a real hardware cursor plane.

Currently, software cursor uses graphics-specific APIs (OpenGL and
QPainter) to paint itself. That will be changed in the future when
rendering parts are extracted from the Scene in a reusable helper.