Use GL_STREAM_READ as we copy from GPU to application.
Use GL_TRIANGLE_STRIP with less vertices.
Remove redundant REDEF.
Free memory returned by glXChooseFBConfig().
Xlib functions may be called directly.
Redirect glXGetFBConfigAttrib(GLX_VISUAL_ID) to DSPL.
Fix get_dimensions(): it may be called only on X drawable, not GLX drawable.
Honor more FB config attributes.