Merge platform+quality into a single quality value

Added another quality setting (VERY HIGH) and mapped all
combinations of platform + the old quality value to the new
4 value quality option, as follows:

desktop + mid/high quality => 3
mobile + high quality => 2
mobile + mid quality, desktop + low quality => 1
mobile + low quality => 0

All choice points in the code which enabled/disabled
graphics features, based on platform + quality,
were mapped onto a table. This showed which features were
disabled or enabled depending on the platform/quality
combination.

It turned out that mid and high quality resulted in the same
settings on desktop, so they could be captured in a single
value (3).

The only change made to the decision points was that desktop +
low quality previously turned off the booster lighting effect,
whereas that effect is now on for desktop + low quality. This was
done so that mobile + mid quality was equivalent to
desktop + low quality.

By doing this, the "mobile" property can be removed. I also
removed the "half" property, and set it based on quality in the
HexGL class (basically, it's enabled for mobile + low quality,
which is new quality value 0).

index.html

@@ -52,7 +52,7 @@
           <div id="start">Start</div>
           <div id="s-controlType">Controls: LeapMotion</div>
           <div id="s-quality">Quality: High</div>
-          <div id="s-platform">Platform: Desktop</div>
+          <div id="s-hud">HUD: On</div>
           <div id="s-godmode">Godmode: On</div>
         </div>
       </div>