r_mirror/portal_recursions in Display Options

[Xeotroid]

I suggest adding a slider for r_mirror_recursions to the Hardware Renderer submenu and a slider for r_portal_recursions to the Software Renderer submenu. While the default value of 4 is pretty much universally a good balance of performance and quality, having the option be there would be nice, especially given the slightly confusing CVar names (it certainly couldn't be me who changed r_mirror_recursions to 10 a long time ago and then recently wondered why a map runs like utter crap and was dumbfounded that r_portal_recursions doesn't seem to do anything).