lavf/matroskaenc: Write Block Keyframe correctly

lavf/matroskaenc: Write Block Keyframe correctly

Per matroska Block Structure [1], for keyframes 0th bit of the flag
should not be set (unlike SimpleBlocks). For Blocks, keyframes is
inferred by the absence of ReferenceBlock element (as done by
matroskadec). This CL writes the flag correctly and inserts the
ReferenceBlock element for non-keyframes. The timestamp inserted is
that of the immediately preceding frame (which is true for VP8 and VP9
- the only 2 codecs using the matroska block element as of now). It
also considers all non-video frames (audio, subtitles, metadata) to
be keyframes.

[1] http://www.matroska.org/technical/specs/index.html#block_structure

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>