console: rewrite console layer

console: rewrite console layer

This is a rewrite of the whole console layer so we can remove the
kmscon_buffer object to speed up the console.

This removes the split between the scroll region and the two margins so we
can resize margins in O(1). This also correctly merges the remaining
console modes so vte does not have to track them.

Btw. vim runs smoothly on kmscon with this console layer now (finally!).

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>