These functions ends-up calling gdk-pixbuf for loading textures/bitmaps
from a file and they don't seem to be used anywhere.
These changes are only useful with the following up commit.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3097>
This basically reverts commit 54735dec, which tried to avoid the
GLib-defined types in favor the standard C ones. One exception to this
is the bool type, for which the commit introduces a new type CoglBool.
Let's just get rid of this type in favor of having consistency with the
GLib types. Note by the way that neither CoglBool nor gboolean (which
has a size of `int`) are completely compatible with bool (size `char`).
https://gitlab.gnome.org/GNOME/mutter/merge_requests/321
