pass timestamp value to output image file name

[ypatios]

hypothetical example:

$ ffmpeg -i input.avi -vf select='eq(pict_type,I)' -vsync 0 -an I_frame_%07pts.png

assuming that the input.avi contains Iframes on every second, the resulting output file names should be as the following:

I_frame_0000000.png
I_frame_0001000.png
I_frame_0002000.png
...

(the timestamp value should be in milliseconds to avoid comma, etc)

[Stefano Sabatini]

This is a duplicate of #1452, closing. Please trac that ticket instead.