Support planar video formats

[fasterthanlime]

Basically:

• originally the encoder code assumed interleaved video format
• so the struct only took a linesize param
• now: in the wire format (messages.fbs), linesize and offset are arrays
• ... but the encoder video format still has just linesize and vflip

So we need to adjust the encoder format struct, see definition here and make the encoder code use it properly.

Also, for OpenGL capture, writevideoformat should take care of the stride vflip trick by itself, see these lines.

In general the encoder code needs a big cleanup but this can be done before or after it independently.