ShaderQualityGroup based 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
[High] = everything on
[Normal] = shuts off SSAO
[Low] = additionally disabled HDR
[Lowest] = takes no prisoners.
bumps the max shader quality level up to highest, and shifts postfx shutoffs upwards for a bit more fine-grained control.
Merge pull request #43 from Azaezel/postfx_shutoffs

Postfx shutoffs