vpx: mux WebP in RIFF container

The goal here is to get snapshots containerized as per: https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossy

instead of purely a VP8 frame (which most players are still happy with).

Some questionable bits, should the vpx video encoder plugin be:

• a) making an image (already the case)
• b) effectively muxing that image (new!)
• c) outputting a chain of block_t of block_t header->(one or more block_t packets of VP8 data) (could've happened before if a frame was split across multiple packets, it's just now a sure thing since the header is prepended as a block_t) this is now done in a single block_t.

c) could be deferred by simply prepending the header in the block_t p_buffer directly (like start writing VP8 data at an offset since the header size is fixed and then update it at the end)...DONE