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/layout.rs

@@ -1,9 +1,6 @@
 //! Where a frame puts things, with no window to put them in.
 
-mod common;
-
-use common::corners;
-use iris::harness::Harness;
+use iris::harness::{Harness, assert_corners};
 use iris::prelude::*;
 
 /// A fixed 100 wide, and the rest of the 400 to its neighbour.
@@ -19,8 +16,8 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     let mut h = Harness::new((400, 200));
     let (left, right) = two_rects(&mut h);
 
-    assert_eq!(corners(&h, &left), (0.0, 0.0, 100.0, 200.0));
-    assert_eq!(corners(&h, &right), (100.0, 0.0, 400.0, 200.0));
+    assert_corners!(h, left, (0, 0), (100, 200));
+    assert_corners!(h, right, (100, 0), (400, 200));
 }
 
 #[test]
@@ -32,6 +29,6 @@ fn resizing_relays_out_against_the_new_output() {
     assert!(h.needs_redraw());
     h.frame();
 
-    assert_eq!(corners(&h, &left), (0.0, 0.0, 100.0, 100.0));
-    assert_eq!(corners(&h, &right), (100.0, 0.0, 800.0, 100.0));
+    assert_corners!(h, left, (0, 0), (100, 100));
+    assert_corners!(h, right, (100, 0), (800, 100));
 }