QuakeSpasm-OpenVR 0.96.0a

Changes:

• Merged QuakeSpasm v0.96.0 (see changes here)
• Fixed black screen on gamma/brightness settings other than 1.0 and extended gamma range so that game can look better on VR headsets with low contrast ratio (many thanks to @ksuprynowicz)
• Changed default of cvar scr_centertime to 4. This makes duration of center messages longer. Previous value was 2.