avformat/lavfi: force probesize to cover at least 30 frames

This ensures that fps and other values are estimated correctly,
the probesize also is intended to limit disk/protocol reads which
does not apply to lavfi inputs at all.

Fixes Ticket1051

Something similar could be usefull to other input devices

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