Wrong sound and delay timer frequency

[theobori]

You decrement the delay and the sound timer in Processor::tick.

But the function is managed by only one clock that you have set to 2 milliseconds, it means the has a 500hz clock. But according to this document http://devernay.free.fr/hacks/chip8/C8TECH10.HTM#2.5 it is supposed to have a 60hz clock.

It impacts the gameplay on some games like Space Invaders, the player moves too slowy compared to the ennemies.