media-libs/gst-plugins-bad: Default enable egl and opengl for webkit-gtk and 21st...

media-libs/gst-plugins-bad: Default enable egl and opengl for webkit-gtk and 21st century, fix USE=egl to matter

webkit-gtk IUSE default enables egl and gstreamer, while now requiring gstreamer[egl] in that
case as well. So try out default enabling egl here to work out of the box, plus opengl so we
don't hit our own newly added constraint of wanting opengl or gles2 with USE=egl (otherwise
USE=egl has no effect, because if neither is defined, the EGL code isn't built at all, as it's
part of the gstgl library).

Package-Manager: Portage-2.3.8, Repoman-2.3.2