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/rendering_test.rs

@@ -419,10 +419,7 @@ impl TextureManager {
             let height = 1;
             ctx.load_texture(
                 "color_test_gradient",
-                epaint::ColorImage {
-                    size: [width, height],
-                    pixels,
-                },
+                epaint::ColorImage::new([width, height], pixels),
                 TextureOptions::LINEAR,
             )
         })