Work around mouse capture SDL bug on macOS

On macOS, SDL relative mouse mode does not work correctly when the
cursor is outside the window.

As a workaround, move the cursor inside the window before setting the
relative mouse mode.

Refs SDL/#5340 <https://github.com/libsdl-org/SDL/issues/5340>
PR #3031 <https://github.com/Genymobile/scrcpy/pull/3031>