lavfi: add frame counter into AVFilterLink and use it in filters.
lavfi: add timeline support.

Flag added in a few simple filters. A bunch of other filters can likely
use the feature as well.
lavfi: add 'enable' command injection to filters supporting timeline.
lavfi/overlay: support timeline through the new system.
avfilter/drawtext: add pict_type support

Fixes Ticket2470

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>