Add a test that HeadlessRenderer is object-safe.

This will cause a faster failure in the event of changes that make it
non-object-safe, rather than only seeing the problem once the build gets
to the `test-renderers` crate.

all-is-cubes/src/camera/renderer.rs

@@ -97,3 +97,10 @@ pub fn info_text_drawable<C: PixelColor + 'static>(
         Baseline::Top,
     )
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn _headless_renderer_is_object_safe(_: &dyn HeadlessRenderer) {}
+}