Fix #5176 by premultiplying the alpha channel to the color channels

This is GIF specific. It's also done when the image is PNG but PNG is
handled separately with the PNG crate, whereas GIFs are handled by the
stb-image crate and the distinction between alpha and non-alpha-supporting
images was missing.

components/net/image/base.rs

@@ -59,7 +59,12 @@ pub fn load_from_memory(buffer: &[u8]) -> Option<Image> {
         match stb_image::load_from_memory_with_depth(buffer, FORCE_DEPTH, true) {
             stb_image::LoadResult::ImageU8(mut image) => {
                 assert!(image.depth == 4);
+                // handle gif separately because the alpha-channel has to be premultiplied
+                if is_gif(buffer) {
+                    byte_swap_and_premultiply(image.data.as_mut_slice());
+                } else {
                     byte_swap(image.data.as_mut_slice());
+                }
                 Some(png::Image {
                     width: image.width as u32,
                     height: image.height as u32,
@@ -77,3 +82,10 @@ pub fn load_from_memory(buffer: &[u8]) -> Option<Image> {
         }
     }
 }
+
+fn is_gif(buffer: &[u8]) -> bool {
+    match buffer {
+        [b'G',b'I',b'F',b'8', n, b'a', ..] if n == b'7' || n == b'9' => true,
+        _ => false
+    }
+}