Flag added in a few simple filters. A bunch of other filters can likely
use the feature as well.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

This is mostly automated global search and replace

The deprecated aconvert filter is disabled, if it still has users
it should be updated

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

This is a port of virtual dub's histogram equalization filter by Donald
A. Graft. Based on the work by Jérémy Tran <tran.jeremy.av@gmail.com>,
done for SOCIS 2012.