It may happen that the clip region becomes empty (for literally invisible items).
For example this happens in the printer demo.
In that case we want to return an empty rect.
When the renderer does not re-implement visit_clip, we call combine_clip.
Then we're missing out on an optimization the GL renderer does: When the resulting clip region
is empty, we do not need to recurse into children for rendering.
That itself reduces the property dependency chain and avoids unnecessary
updates when invisible (clipped) children change properties.
Split backend-gl-* into eventloop-winit-* and renderer-femtovg.
The old feature names are still available.
For consistency eventloop-qt and renderer-qt are aliases for backend-qt.
