Fix sometimes blurry SVGs (#7071)

* Closes https://github.com/emilk/egui/issues/3501

The problem occurs when you want to render the same SVG at different
scales, either at the same time in different parts of your UI, or at two
different times (e.g. the DPI changes).

The solution is to use the `SizeHint` as part of the key.

However, when you have an SVG in a resizable container, that can lead to
hundreds of versions of the same SVG. So new eviction code is added to
handle this case.

crates/emath/src/ordered_float.rs

@@ -21,6 +21,13 @@ impl<T: Float + Copy> OrderedFloat<T> {
     }
 }
 
+impl<T: std::fmt::Debug> std::fmt::Debug for OrderedFloat<T> {
+    #[inline]
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 impl<T: Float> Eq for OrderedFloat<T> {}
 
 impl<T: Float> PartialEq<Self> for OrderedFloat<T> {