Allow changing of crosshair size + fixes

* vr_crosshair_size allows setting size of vr crosshairs from 1 to 5
points diameter
* Added VR_SetAngels() to allow properly setting view angles without
messing with aim modes
* Now retain entire last orientation and aim vectors
* Fix issues related to intermission direction with some aim modes
* Fix issues related to resetting HMD orientation with certain aim modes
Only draw crosshair if player is alive
Support pre-configured magnetic drift correction

-Also fixed an issue with the right eye view offset not being set
correctly
Merge branch 'master' of https://github.com/phoboslab/Quakespasm-Rift
Update README.md to cover vr_crosshair_size
Fix in auto magnetic calibration

Fixed not being able to disable auto magnetic calibration. Also, some
really bad pointer handling on my part.
Yaw correction fixes related to LibOVR 0.2.4

-Doesn't clear calibration data when disabled (prevents need to
autocalibrate again)
-Enables/disables yaw correction properly