Some flac muxers write truncated metadata picture size if the picture
data do not fit in 24 bits. Detect this by truncting the size found inside
the picture block and if it matches the block size use it and read rest
of picture data.

This workaround is only for flac files and not ogg files with flac
METADATA_BLOCK_PICTURE comments and it can be disabled with strict level
above normal. Currently there is a 500MB limit on truncate size to protect
from large memory allocations.

The truncation bug in lavf flacenc was fixed in e447a4d1
but based on existing broken files other unknown flac muxers seems to truncate also.
Before the fix a broken flac file for reproduction could be generated with:
ffmpeg -f lavfi -i sine -f lavfi -i color=red:size=2400x2400 -map 0:0 -map 1:0 -c:v:0 bmp -disposition:1 attached_pic -t 1 test.flac

Fixes ticket 6333
Signed-off-by: Anton Khirnov <anton@khirnov.net>