darwin: do not create double-buffered offscreen pixel formats

http://xquartz.macosforge.org/trac/ticket/536

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>

src/glx/apple/apple_visual.c

@@ -116,7 +116,7 @@ apple_visual_create_pfobj(CGLPixelFormatObj * pfobj, const struct glx_config * m
       *uses_stereo = false;
    }
 
-   if (mode->doubleBufferMode) {
+   if (!offscreen && mode->doubleBufferMode) {
       attr[numattr++] = kCGLPFADoubleBuffer;
       *double_buffered = true;
    }