[Xastir] No more lockups with radar map

[Alex Carver]

Switching to GraphicsMagick worked but it also explained the error.  ImageMagick was failing without printing an error so I had no idea what was going on.  But with GM, it tells me that occasionally the radar GIF is corrupt and thus GM bombs out.  It would appear that the combination of IM/Xastir doesn't handle this situation quite as gracefully as GM/Xastir thus leading to the lockups.