add a way to enable/disable video effects via libvlc

Currently, enabling/disabling video effects is done running var_setstring to vout (e.g. var_SetString( p_vout, "video-filter", "invert"); ), except this (and vout) isn't available from external apps using libvlc (like the photobooth exemple).

It would be necessary to add a way to enable/disable video effects via libvlc.

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