Improve the compressibility of the index files a bit

While the space occupied on disk is utterly irrelevant, this speeds up
writing the index file a bit (as much as twice as fast for some Matroska
files).
Allow negative linesizes

libavcodec appears to use the convention of negative strides for images
stored bottom-to-top, so don't reject them when verifying that it
actually gave us data.

Closes #102.