Return Unimplemented error on other platforms

Add line to docs about unsupported platforms

Test fixes

src/cg.rs

@@ -72,7 +72,7 @@ impl CGImpl {
 
     /// Fetch the buffer from the window.
     pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
-        todo!()
+        Err(SoftBufferError::Unimplemented)
     }
 }
 

src/error.rs

@@ -38,6 +38,9 @@ pub enum SoftBufferError {
 
     #[error("Platform error")]
     PlatformError(Option<String>, Option<Box<dyn Error>>),
+
+    #[error("This function is unimplemented on this platform")]
+    Unimplemented,
 }
 
 /// Convenient wrapper to cast errors into SoftBufferError.

src/lib.rs

@@ -320,6 +320,7 @@ impl Surface {
     /// ## Platform Dependent Behavior
     ///
     /// - On X11, the window must be visible.
+    /// - On macOS, Redox and Wayland, this function is unimplemented.
     pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
         self.surface_impl.fetch()
     }

src/orbital.rs

@@ -146,7 +146,7 @@ impl OrbitalImpl {
 
     /// Fetch the buffer from the window.
     pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
-        todo!()
+        Err(SoftBufferError::Unimplemented)
     }
 }
 

src/wayland/mod.rs

@@ -132,7 +132,7 @@ impl WaylandImpl {
 
     /// Fetch the buffer from the window.
     pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
-        todo!()
+        Err(SoftBufferError::Unimplemented)
     }
 }
 

tests/present_and_fetch.rs

@@ -23,6 +23,7 @@ fn all_red(elwt: &EventLoopWindowTarget<()>) {
     }
 
     // winit does not wait for the window to be mapped... sigh
+    #[cfg(not(target_arch = "wasm32"))]
     std::thread::sleep(std::time::Duration::from_millis(1));
 
     let context = unsafe { Context::new(elwt) }.unwrap();
@@ -43,7 +44,10 @@ fn all_red(elwt: &EventLoopWindowTarget<()>) {
     buffer.present().unwrap();
 
     // Check that all pixels are red.
-    let screen_contents = surface.fetch().unwrap();
+    let screen_contents = match surface.fetch() {
+        Err(softbuffer::SoftBufferError::Unimplemented) => return,
+        cont => cont.unwrap(),
+    };
     for pixel in screen_contents.iter() {
         assert_eq!(*pixel, 0xFF000000);
     }