connection, client: Avoid locale-dependent float printing

Specifically, in the log formed when WAYLAND_DEBUG is set, this commit
ensures that floating point numbers are formatted using '.' instead of
the locale-specific decimal separator. As the debug logs are not
otherwise localized for end-users, and may be parsed by scripts, it is
better to have consistent output here.

The 24.8 fixed point numbers are now represented with 8 digits after
the decimal, since this is both exact and simpler to compute.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>