Bug 1329574 - try to create double-buffer for GLX gl context. r=jrmuizel?

Some FBConfigs do not have the double-buffer attribute. This patch adds one
more selection condition for the double-buffer.

MozReview-Commit-ID: 6QLpEXAi04X

gfx/gl/GLContextProviderGLX.cpp

@@ -1259,6 +1259,7 @@ GLContextGLX::FindFBConfigForWindow(Display* display, int screen, Window window,
     } else {
         const int webrenderAttribs[] = {
             LOCAL_GLX_DEPTH_SIZE, 24,
+            LOCAL_GLX_DOUBLEBUFFER, True,
             0
         };