Improve multi-viewports across monitors with different scales (#3663)

It required some ugly code in `egui-winit` in order to fix this without
breaking sem-ver (I want this to land in a 0.24.1 patch release). I'll
clean up in time for 0.25.

There is still a font rendering bug when using immediate viewports
across multiple viewports, but that's harder to fix.

crates/epaint/src/util/ordered_float.rs

@@ -55,6 +55,13 @@ impl<T: Float> Hash for OrderedFloat<T> {
     }
 }
 
+impl<T> From<T> for OrderedFloat<T> {
+    #[inline]
+    fn from(val: T) -> Self {
+        OrderedFloat(val)
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// Extension trait to provide `ord()` method.