In debug logging, print 'fixed' values as floats

[colinmarc]

I got a bit confused by the following:

2024-04-17T19:51:21.930954Z DEBUG session{session_id=365288}: mmserver::compositor: wheel delta: 0.0 -48.0
[ 768472.301][rs] -> [email protected]_source(2)
[ 768472.333][rs] -> [email protected](13025, 0, -12288)

[ids1024]

This seems to match wayland-backend/client_system, except libwayland prints more than 4 digits.

Truncating to 4 digits makes sense given it doesn't have that much precision anyway.

I guess this should have a CHANGELOG entry.

[elinorbgr]

LGTM, with a changelog entry as pointed by @ids1024

[ids1024]

Merged with a changelog entry as part of #732. Thanks!