[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>

Documentation/media/uapi/v4l/pixfmt-nv12mt.rst

@@ -36,7 +36,7 @@ Layout of macroblocks in memory is presented in the following figure.
 .. _nv12mt:
 
 .. figure::  pixfmt-nv12mt_files/nv12mt.*
-    :alt:    nv12mt.gif
+    :alt:    nv12mt.png
     :align:  center
 
     V4L2_PIX_FMT_NV12MT macroblock Z shape memory layout
@@ -53,7 +53,7 @@ interleaved. Height of the buffer is aligned to 32.
 .. _nv12mt_ex:
 
 .. figure::  pixfmt-nv12mt_files/nv12mt_example.*
-    :alt:    nv12mt_example.gif
+    :alt:    nv12mt_example.png
     :align:  center
 
     Example V4L2_PIX_FMT_NV12MT memory layout of macroblocks