SGIX_blend_alpha_minmax and EXT_blend_logic_op extensions. It is
tested with the osgblendequation example. If the extensions are not
supported, a WARN level notification is generated."
Note, this required adding a unsigned int context ID to the osg::isGLUExtensionSupported(,)
and osg::isGLExtensionSupported(,) functions. This may require reimplementation
of end user code to accomodate the new calling convention.
