I added support for chromatic aberration correction

A few notes about it:
-It is disabled by default because it, for some reason, tends to mangle the text. It shouldn't since it is applied radially from the center of the lens, but it winds up having that effect anyways. I'm not entirely positive why.
-It can be enabled/disabled using the r_oculusrift_chromabr cvar. 1 enabled, 0 disables.
-It forces a full re-initialization of the HMD renderer. I did this so that shader selection was only performed once (at init) instead of having to test every frame. It should be able to be tweaked to support switching without performing a reinitialization, although it would either require either being able to initialization a shader outside of R_InitHMDRenderer *or* just initializing both and then adjusting the pointer on the fly. I don't know if you had a preference or not, so I didn't implement it.

Thanks!