Implement bindings for debug output feature

This patch implements ARB_debug_output/KHR_debug. It's was an extension
before and now it's in core since OpenGL 4.3.

Interface is not totally match C version:
 - no userParam passed to glDebugMessageCallback();
 - glGetDebugMessageLog() imeplemented as GetNextDebugMessage();

Any suggestions about interface or implementation are welcome.