Fixed broken test for broken RandR gamma.

src/x11_gamma.c

@@ -54,7 +54,7 @@ void _glfwInitGammaRamp(void)
         XRRScreenResources* rr = XRRGetScreenResources(_glfw.x11.display,
                                                        _glfw.x11.root);
 
-        if (XRRGetCrtcGammaSize(_glfw.x11.display, rr->crtcs[0]))
+        if (XRRGetCrtcGammaSize(_glfw.x11.display, rr->crtcs[0]) == 0)
         {
             // This is probably older Nvidia RandR with broken gamma support
             // Flag it as useless and try Xf86VidMode below, if available