Add configurable effect intervals

[Mikusch]

Effects can now define the rate at which they update. This both adds more control and heavily reduces CPU usage by not having to update several effects every frame.

The default value is defined in sm_chaos_effect_update_interval (def: 0.1). The update interval can be defined in the return value of the update function:

function ChaosEffect_Update()
{
    // Update in `sm_chaos_effect_update_interval` seconds
    if (someCondition)
        return null

    // Update in 0.5 seconds
    if (someOtherCondition)
        return 0.5

    // Update next frame
    return -1
}

• SourcePawn effects that return void or a float value of 0.0 will use the default value
• VScript effects that return null will use the default value

To guarantee execution on the next frame for any effect type, return -1.