avcodec/xbmenc: xbm Lower memory use

avcodec/xbmenc: xbm Lower memory use

Two minor memory improvements.

First bug reduces memory needed to about 6/7 the needed amount, which
allows you to host almost 7 pictures in the same memory needed for 6
Second is a recalculation of the total additional memory for headers etc.

size = avctx->height x (linesize * 6 + 1) + (31+32+38+4+1)
Subject: [PATCH 2/3] avcodec/xbmenc: xbm Lower memory use

Small 6/7th size memory reduction.
size = avctx->height x (linesize * 6 + 1) + (31+32+38+4+1)

Signed-off-by: Joe Da Silva <digital@joescat.com>