Fix build error, missing return value

Returning 0 on error. This caused a build break on OS X at least.
Fix build warnings

Tested on OS X, with LLVM 5.0
Drop frames to keep up with the OPC source

OpenGL rendering is often limited to the vertical refresh rate of the monitor, whereas OPC sources can send pixel data much faster than this.  This can cause data to backlog in the TCP socket, leading to lots of latency.

This patch drains any queued frames every time it receives, and displays only the most recent frame.
Turn off verbose output

The debug output on stdout was getting way too chatty for my tastes, and it seemed like a performance liability as well. Turning it off, but in a way that makes it easy to turn on later with some kind of config option when we decide how to implement that.