Support for debug contexts (Win32 and Linux only)

ContextSettings::debug will result in a debug context on Win32 and Linux.
MacOS seems to have no method of creating a debug context, so it does nothing there.

As per above, it's impossible to support this under MacOS, but the functionality is useful enough elsewhere (and I'd rather not maintain a fork).

Additionally, it might be an idea to add an err() for attempting to use this with a major version below 3, and on OS X.