Change ingame timer from jiffies to milliseconds

[shenef]

Currently the ingame timer runs in "jiffies", a 60th of a second.
Changing that to milliseconds would allow the Speedrun/Time Attack community to have more accurate readings on the times they achieve.
Many levels already have very competitive times and therefore there are many tied times. Reaching new records has become very hard since every step up happens only in a 16.66ms interval.

Are there any performance concerns? stepping up the physics from 60 times per second to 1000 times per second has to have a performance impact