Compare regions, not four loose numbers

`PixelRegion` derives `PartialEq`, `Clone` and `Copy`, so `assert_corners!`
compares one against another instead of flattening both to a tuple whose
order there was nothing to check. A failure now prints two regions.

core/src/orientation/pos.rs

@@ -421,7 +421,7 @@ impl Display for UiRegion {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct PixelRegion {
     pub top_left: Vec2,
     pub bot_right: Vec2,