GLX_SGI_make_current_read
http://www.opengl.org/registry/specs/SGI/make_current_read.txt
GLX_SGI_make_current_read
	GLXDrawable glXGetCurrentReadDrawableSGI (void)
	Bool glXMakeCurrentReadSGI (Display* dpy, GLXDrawable draw, GLXDrawable read, GLXContext ctx)
