specs: Add EGL_MESA_device_software

specs: Add EGL_MESA_device_software

The device extension string is expected to contain the name of the
extension defining what kind of device it is, so the caller can know
what kinds of operations it can perform with it. So that string had
better be non-empty, hence this trivial extension.

v2:
 - drop "fallback", update history and update contributor list

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>