fix wrong capture result if playback is paused
videocapture: capture current displayed frame. code clean up. api change

old implemention was the next frame will be captured.
examples: add video capture example
videocapture: add convenience signal to get QImage
videocapture: wait for done if app is exiting.
VideoFrame.clone() only copy properties if a frame has no valid data
videocapture: check frame data. fix qimage not created if no sav
dxva: check ffmpeg runtime support for hevc

currently hevc dxva2 is not supported by ffmpeg. only lavfilters ffmpeg
support
subtile: set correct pts & duration for ffmpeg>=2.5 ass decoder

otherwise the decoded duration is wrong. ass decoder changed
add ffmpeg decoder base class. restrict base ctor as private
mac: add libs to link against static ffmpeg
Merge remote-tracking branch 'upstream/master' into bm2