lavfi: Increase drawtext font size limits

Raise the maximum font size to a larger value.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
latm: Do not give a score for a single instance

Bug-Id: 773
CC: libav-stable@libav.org
mp3: Tweak the probe scores

Having more than 10 consecutive frames decoded as mp3 should be
considered a clear signal that the sample is mp3 and not mpegps.

Reported-By: Florian Iragne <florian@iragne.fr>
CC: libav-stable@libav.org
doc: Use the correct @subsection tag

makeinfo would otherwise refuse to build it.