#58 allow control over error/warning message output

[Chris Marshall]

In particular, I would like to disable the helpful
fgWarning messages such as:

"freeglut (myprogram): Unable to create direct context rendering for window 'Test image monochrome' This may hurt performance."

which is useful the first time but gets old
quickly. It should be possible to disable
warnings and/or errors. A mask could make
category based control possible if desired.

Thanks.

[Sven Panne]

This warning has simply been removed, it was annoying. On some platforms there is simply no such thing as a direct context, so it wasn't helpful at all.