st/glx: accept GLX_SAMPLE_BUFFERS/SAMPLES_ARB == 0

Only fail if GLX_SAMPLE_BUFFERS_ARB or GLX_SAMPLES_ARB are non-zero.
We were already doing this in the older swrast/glx code.

This fixes a piglit/waffle problem where we'd always fail to get a
visual/config and report the test as "skip".

Note: This is a candidate for the stable branches.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit c6d74bfaf6)

src/gallium/state_trackers/glx/xlib/glx_api.c

@@ -832,11 +832,13 @@ choose_visual( Display *dpy, int screen, const int *list, GLboolean fbConfig )
           * GLX_ARB_multisample
           */
          case GLX_SAMPLE_BUFFERS_ARB:
-            /* ms not supported */
-            return NULL;
          case GLX_SAMPLES_ARB:
-            /* ms not supported */
-            return NULL;
+            parselist++;
+            if (*parselist++ != 0) {
+               /* ms not supported */
+               return NULL;
+            }
+            break;
 
          /*
           * FBConfig attribs.