One proxy, and a macro instead of a shared test module

`Proxy` is the task queue as well as the way an application sends its own
events, so `ProxyQueue` is gone. `schedule_redraw` becomes
`request_redraw_if_needed`, which says what the comment beside it was
saying.

`assert_corners!` replaces the region helper, so `tests/common` goes with
it, and the scroll test now states both corners rather than one number.

tests/scroll.rs

@@ -1,9 +1,6 @@
 //! Scrolling moves content and stops at its ends.
 
-mod common;
-
-use common::corners;
-use iris::harness::Harness;
+use iris::harness::{Harness, assert_corners};
 use iris::prelude::*;
 
 #[test]
@@ -16,14 +13,14 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     h.move_to((200, 100));
 
     // `Scroll` starts snapped to the end.
-    assert_eq!(corners(&h, &top).1, -200.0);
+    assert_corners!(h, top, (0, -200), (400, 0));
 
     // The handler scales a wheel line by 50.
     h.scroll((0, 1));
     h.frame();
-    assert_eq!(corners(&h, &top).1, -150.0);
+    assert_corners!(h, top, (0, -150), (400, 50));
 
     h.scroll((0, 10));
     h.frame();
-    assert_eq!(corners(&h, &top).1, 0.0);
+    assert_corners!(h, top, (0, 0), (400, 200));
 }