vp8: document handling of corrupt frames

vpx_codec_decode() can return VPX_CODEC_OK for a corrupted VP8 frame,
e.g. when data is missing from the last token partition. Document that
callers must query VP8D_GET_FRAME_CORRUPTED after every decode.

Bug: chromium:561396583
Change-Id: I07f1e495e81a1ff51bf2502b7a64b8c1744bb764
2 files changed