video: add metadata handling for spherical video

This adds handling of spherical video metadata: retrieving it from
demux_lavf and demux_mkv, passing it through filters, and adjusting it
with vf_format. This does not include support for rendering this type of
video.

We don't expect we need/want to support the other projection types like
cube maps, so we don't include that for now. They can be added later as
needed.

Also raise the maximum sizes of stringified image params, since they
can get really long.

TOOLS/matroska.py

@@ -137,6 +137,13 @@ elements_matroska = (
                             'LuminanceMin,            55DA, float',
                         ),
                     ),
+                    'Projection, 7670, sub', (
+                        'ProjectionType, 7671, uint',
+                        'ProjectionPrivate, 7672, binary',
+                        'ProjectionPoseYaw, 7673, float',
+                        'ProjectionPosePitch, 7674, float',
+                        'ProjectionPoseRoll, 7675, float',
+                    ),
                 ),
                 'Audio, e1, sub', (
                     'SamplingFrequency, b5, float',