igv: 2.4.6 -> 2.4.8

Semi-automatic update. These checks were performed:

- built on NixOS
- found 2.4.8 with grep in /nix/store/3vwg5hpprkm3v345fy05gxdjicfakql7-igv-2.4.8
- found 2.4.8 in filename of file in /nix/store/3vwg5hpprkm3v345fy05gxdjicfakql7-igv-2.4.8

cc "@mimadrid"