Hide SoftwareRenderer behind feature flag

- Don't let backend depend on `CursorBackend` (used to be `GraphicsBackend`) anymore

src/backend/graphics/mod.rs

@@ -12,4 +12,5 @@ pub use self::cursor::*;
 pub mod gl;
 #[cfg(feature = "renderer_glium")]
 pub mod glium;
+#[cfg(feature = "renderer_software")]
 pub mod software;

src/backend/graphics/software.rs

@@ -1,11 +1,10 @@
 //! Common traits and types used for software rendering on graphics backends
 
-use super::GraphicsBackend;
 use std::error::Error;
 use wayland_server::protocol::wl_shm::Format;
 
 /// Trait that describes objects providing a software rendering implementation
-pub trait CpuGraphicsBackend<E: Error>: GraphicsBackend {
+pub trait CpuGraphicsBackend<E: Error> {
     /// Render a given buffer of a given format at a specified place in the framebuffer
     ///
     /// # Error