Extend value range for SDK mouse scrolling

SDL precise scrolling can sometimes produce values greater than 1 or
less than -1.

On the wire, the value is encoded as a 16-bit fixed-point number.

Previously, the range was interpreted as [-1, 1], using 1 bit for the
integral part (the sign) and 15 bits for the fractional part.

To support larger values, interpret the range as [-16, 16] instead,
using 5 bits for the integral part and 11 bits for the fractional part
(which is more than enough).

PR #6172 <https://github.com/Genymobile/scrcpy/pull/6172>