[C#] Remove 'readonly' from struct and field definitions

[andreakarasho]

The host pass a vector of Velocity to the guest.
You want to change only the X.
Today you have to re-assign each index like:

readonly struct Velocity { public readonly float X, Y; }
...
velocities[i] = new Velocity(velocities[i].X + 1, velocities[i].Y);

Removing the readonly you can simply do:

struct Velocity { public float X, Y; }
...
velocities[i].X += 1;