docs-rst: nv12mt zigzag images: replace by SVG images

Instead of using bitmap images to show the zigzag macroblock
parsing, replace it by a SVG ones, with is scalable.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>