fix build against ffmpeg >= 1.0

these commits were taken from xbmc (lib/DllAvFilter.h) :
 AvFilter wrapper: Remove unused avfilter_poll_frame which has been removed in libavfilter 3.
 AvFilter wrapper: Remove unused avfilter_request_frame mapping as the function has been removed in libavfilter 3.
 AvFilter wrapper: Remove unused avfilter_get_video_buffer mapping as the function has been removed in libavfilter 3.
 AvFilter: Map and use av_buffersrc_add_frame instead of av_vsrc_buffer_add_frame with libavfilter version 3.
Merge pull request #101 from twolife/master

fix build against ffmpeg >= 1.0