Overlay/Blend surface generator API
This new API allows the user (vppoverlay) to call a single method (process) which handles the [display] lock/unlock logic internally for them.
This API supersedes the risky begin, render, end API. It eliminates the need for the user to call a lock method (process_begin) before processing the input buffers (process_render) and calling an unlock method (process_end) afterwards.
Resolves #219 (closed)