Track original SVG size (#7098)

This fixes bugs related to how an `Image` follows the size of an SVG.

We track the "source size" of each image, i.e. the original width/height
of the SVG, which can be different from whatever it was rasterized as.

crates/egui_demo_lib/src/demo/tests/svg_test.rs

@@ -29,10 +29,11 @@ impl crate::View for SvgTest {
         ui.color_edit_button_srgba(color);
         let img_src = egui::include_image!("../../../data/peace.svg");
 
-        // First paint a small version…
-        ui.add_sized(
-            egui::Vec2 { x: 20.0, y: 20.0 },
-            egui::Image::new(img_src.clone()).tint(*color),
+        // First paint a small version, sized the same as the source…
+        ui.add(
+            egui::Image::new(img_src.clone())
+                .fit_to_original_size(1.0)
+                .tint(*color),
         );
 
         // …then a big one, to make sure they are both crisp