AP_Camera: runcam backend

[andyp1per]

[rmackay9]

I like where this is going!

[peterbarker]

@andyp1per where are you going with this one?

What advantages were you seeing from being a camera backend?

I'd note that the camera library, even in its most compact form is still going to be pretty chunky, and recent commits have shown we don't really care about keeping it absolutely tight space-wise.

Definitely not saying it's a bad idea!

[andyp1per]

The main thing is to get the mavlink and other support for free. People keep asking for runcam mission support