non-empty attrib_list for glXCreateContextAttribsARB

empty list is legal but causes bogus context on at least i915

BUG: 321843
REVIEW: 111355
FIXED-IN: 4.11

glxbackend.cpp

@@ -169,6 +169,8 @@ bool GlxBackend::initRenderingContext()
         };
 
         const int attribs_legacy[] = {
+            GLX_CONTEXT_MAJOR_VERSION_ARB,               1,
+            GLX_CONTEXT_MINOR_VERSION_ARB,               2,
             0
         };