cogl: Set LOD bias to -0.5 for single mipmap modes

So that whenever forced to choose between two levels of detail (two mipmaps)
we will land on the sharpest looking one (highest resolution). That's the
mipmap level equal to the floor of the current level of detail requested.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5920
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2644>

(cherry picked from commit cc19547b)