Add adjust filter features to libvlc_media_player/libvlc_video

Add adjust filter features to libvlc_media_player/libvlc_video

This patch exposes the adjust module to libvlc_media_player/libvlc_video
and adds a frequently requested featureset: to manipulate brightness,
contrast, gamma, saturation and hue through libvlc. The patch introduces
a new set of functions analogous to
libvlc_video_{get,set}_{marq,logo}_*, hence the use of the function set
should be self explanatory.

New Functions:

* value = libvlc_video_get_adjust_int( p_mi, option );
* libvlc_video_set_adjust_int( p_mi, option, value );
* value = libvlc_video_get_adjust_float( p_mi, option );
* libvlc_video_set_adjust_float( p_mi, option, value );

New Options:

* libvlc_adjust_Enable
* libvlc_adjust_Contrast
* libvlc_adjust_Brightness
* libvlc_adjust_Hue
* libvlc_adjust_Saturation
* libvlc_adjust_Gamma

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>