core: Recommended png depth value 24 instead of 32
core: Recommended png depth value 24 instead of 32

The depth of a PNG is defined excluding the alpha channel so we really have
3x8 bits here.

https://bugzilla.gnome.org/show_bug.cgi?id=705084

GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,jku/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel