mint-arena: Track digital and analog key state fractions separately
mint-arena: Use separate cvars for digital and analog angle turn speed

The default turn speed was too slow. Increasing it makes aiming using
keyboard a lot harder. So use separate cvars for analog and digital.

Using analog and digital at the same time now adds both. This is just
a side effect and doesn't matter enough to fix.
mint-arena: Make joystick analog fraction be 0 at threshold

Scale the axis value so that edge of 'dead zone' aka threshold is 0.
Makes aiming using analog stick a little better (feels more sensitive).