Move mesa dependencies from gnome-session to gnome

One reason for this MR is that mesa and mesa-dri-gallium belong to gnome package better, as GNOME as a desktop recommends 3D acceleration. Alpine's wiki recommends installing this package to install the GNOME desktop, so this should be the way to warranty that the 3D acceleration is present.

Another reason is that gnome-session has an option that allows disabling the acceleration check and running without hardware or software acceleration. It is not the default way to run GNOME, but it might be the case in other environments, e.g: phosh actually runs gnome-session with --disable-acceleration-check flag.