Add mb_info API for signalling constant macroblocks

Add mb_info API for signalling constant macroblocks
Some use-cases of x264 involve encoding video with large constant areas of the frame.
Sometimes, the caller knows which areas these are, and can tell x264.
This API lets the caller do this and adds internal tracking of modifications to macroblocks to avoid problems.
This is really only suitable without B-frames.
An example use-case would be using x264 for VNC.