mesa: return the correct value for GroupStackDepth

We already have one group (the default) as specified in the spec. So
lets return its size, rather than the index of the current group.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/mesa/main/errors.c

@@ -775,7 +775,7 @@ _mesa_get_debug_state_int(struct gl_context *ctx, GLenum pname)
          debug->Log.Messages[debug->Log.NextMessage].length : 0;
       break;
    case GL_DEBUG_GROUP_STACK_DEPTH:
-      val = debug->CurrentGroup;
+      val = debug->CurrentGroup + 1;
       break;
    default:
       assert(!"unknown debug output param");