server_list: fix pre-1.13 favicon data, #353 (#354)

The server list ping response contains a base64-encoded favicon image,
which in pre-1.13 servers can have embedded newlines:
https://wiki.vg/Server_List_Ping#Response

> The favicon should be a PNG image that is Base64 encoded (without
> newlines: \n, new lines no longer work since 1.13) and prepended
> with data:image/png;base64,.

If this was the case, base64 decode would fail with a DecodeError,
similar to Invalid byte (76, 10). We now strip the whitespace, so the
base64 favicon can be decoded properly.

src/screen/server_list.rs

@@ -266,6 +266,8 @@ impl ServerList {
                         format::convert_legacy(&mut desc);
                         let favicon = if let Some(icon) = res.0.favicon {
                             let data_base64 = &icon["data:image/png;base64,".len()..];
+                            let data_base64: String =
+                                data_base64.chars().filter(|c| !c.is_whitespace()).collect();
                             let data = base64::decode(data_base64).unwrap();
                             Some(image::load_from_memory(&data).unwrap())
                         } else {