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_glow/src/painter.rs

@@ -696,10 +696,7 @@ impl Painter {
         for row in pixels.chunks_exact((w * 4) as usize).rev() {
             flipped.extend_from_slice(bytemuck::cast_slice(row));
         }
-        egui::ColorImage {
-            size: [w as usize, h as usize],
-            pixels: flipped,
-        }
+        egui::ColorImage::new([w as usize, h as usize], flipped)
     }
 
     pub fn read_screen_rgb(&self, [w, h]: [u32; 2]) -> Vec<u8> {