Limit number of blood splats on screen

The "bloodsplats" setting has been renamed to "bloodsplatstotal" (still
"unlimited" by default), and a new "bloodsplatsvisible" has been
introduced. It is 1024 by default. This stops slowdown when an excessive
amount of blood splats, such as reported here:
http://www.doomretro.com/2014/05/doom-retro-v142.html#disqus_thread