* refactored lookahead code to compute everything in dda steps
* new 12bits fixed point implementation of 1/sqrt(x)
* replaced RAMPING iterative scheme with an approximation of the explicit formula (sqrt(n+1)-sqrt(n) ~ 1/(2*sqrt(n)))
NB: the 1/sqrt(x) implementation is a bit faster than a 32bits divide (it takes about 11% less time to complete) and could be even faster if one requires only 8bits
(precision starts getting poor for big values of n which are likely to be required by small acceleration values)
fixed first step not being computed before timer is set resulting in wrong first step
fixed an ugly overflow in int_inv_sqrt (was working on x86 because uint32_t cast was implicit)
NB: this fixes acceleration when ACCELERATION_RAMPING is enabled