package/libgl: introduce as a new virtual package for full-openGL providers

Currently, only mesa3d is such a full-openGL provider.

But it is easy to see a few more cropping up at the door:
  - NVidia's binary blob
  - ATI's fglrx binary blob

Having this virtual full-openGL package will help when those are getting in.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <berndkuhls@hotmail.com>
Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>