Make with_x11_visual take ID instead of a pointer

At the moment, the with_x11_visual function takes a pointer and
immediately dereferences it to get the visual info inside. As it is safe
to pass a null pointer to this function, it is unsound. This commit
replaces the pointer parameter with a visual ID, and then uses that ID
to look up the actual visual under
the X11 setup. As this is what was already practically happening before,
this change shouldn't cause any performance downgrades.

This is a breaking change, but it's done in the name of soundness so it
should be okay. It should be trivial for end users to accommodate it,
as it's just a matter of getting the visual ID from the pointer to the
visual before passing it in.

Signed-off-by: John Nunley <dev@notgull.net>

src/platform_impl/linux/x11/mod.rs

@@ -886,6 +886,9 @@ pub enum X11Error {
 
     /// Got an invalid activation token.
     InvalidActivationToken(Vec<u8>),
+
+    /// Could not find a matching X11 visual for this visualid
+    NoSuchVisual(xproto::Visualid),
 }
 
 impl fmt::Display for X11Error {
@@ -902,6 +905,13 @@ impl fmt::Display for X11Error {
                 "Invalid activation token: {}",
                 std::str::from_utf8(s).unwrap_or("<invalid utf8>")
             ),
+            X11Error::NoSuchVisual(visualid) => {
+                write!(
+                    f,
+                    "Could not find a matching X11 visual for ID `{:x}`",
+                    visualid
+                )
+            }
         }
     }
 }