Postfx shutoffs

Adds:
$pref::Video::disableSSAO = false;
$pref::Video::disableHDR = false;
$pref::Video::disableLightRays = false;
$pref::Video::disableDOF = false;
$pref::Video::disableVignette = false;

as new prefs, which can override per map postfx settings
[Highest] = everything on
[High] = shuts off SSAO
[Normal] = additionally disables HDR
[Low] = for new flags is the same as normal, keeps the older shutoffs.
[Lowest] = takes no prisoners.