This commit is the analog of 3f11eac7
for decoding: It sets the AV_FRAME_FLAG_KEY and (for video decoders)
also pict_type to AV_PICTURE_TYPE_I. It furthermore stops setting
audio frames as always being key frames -- it is wrong for e.g.
TrueHD/MLP. The latter also affects TAK and DFPWM.

The change already improves output for several decoders where
it has been forgotten to set e.g. pict_type like speedhq, wnv1
or tiff. The latter is the reason for the change to the exif-image-tiff
FATE test reference file.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>