[media] pixfmt-nv12mt.rst: use PNG instead of GIF

[media] pixfmt-nv12mt.rst: use PNG instead of GIF

pdflatex doesn't like gif images:
	None:None: WARNING: no matching candidate for image URI u'media/uapi/v4l/pixfmt-nv12mt_files/nv12mt.*'
	None:None: WARNING: no matching candidate for image URI u'media/uapi/v4l/pixfmt-nv12mt_files/nv12mt_example.*'

But it works fine with png. So, convert them. As a plus, PNG images
are smaller.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>