video: hardware acceleration

Contains code common to all hardware accelerations to be implemented.

Enables the VAAPI acceleration for intel linux systems, with support
for H.264, H.263 and MPEG4.

To use VAAPI, you need libva, libva-x11 and libva-drm.

Hardware acceleration is enabled by default, and can be disabled with
"./configure --disable-accel".

Change-Id: Id0696465b785de0735bbce9750932ac38efe0713
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>