Fix HID mouse support with SDL precise scrolling

Over HID, only integral scroll values can be sent. When SDL precise
scrolling is active, scroll events may include fractional values (e.g.,
0.05), which are truncated to 0 in the HID event.

To fix the problem, use the integral scroll value reported by SDL, which
internally accumulates fractional deltas.

Fixes #6156 <https://github.com/Genymobile/scrcpy/issues/6156>
PR #6172 <https://github.com/Genymobile/scrcpy/pull/6172>