Moved RandR gamma probe to init.

src/x11_init.c

@@ -475,6 +475,8 @@ static GLboolean initExtensions(void)
 
     if (_glfw.x11.randr.available)
     {
+        XRRScreenResources* sr;
+
         if (!XRRQueryVersion(_glfw.x11.display,
                              &_glfw.x11.randr.versionMajor,
                              &_glfw.x11.randr.versionMinor))
@@ -490,6 +492,21 @@ static GLboolean initExtensions(void)
         {
             _glfw.x11.randr.available = GL_FALSE;
         }
+
+        sr = XRRGetScreenResources(_glfw.x11.display, _glfw.x11.root);
+
+        if (!sr->ncrtc || !XRRGetCrtcGammaSize(_glfw.x11.display, sr->crtcs[0]))
+        {
+            // This is either a headless system or an older Nvidia binary driver
+            // with broken gamma support
+            // Flag it as useless and fall back to Xf86VidMode gamma, if
+            // available
+            _glfwInputError(GLFW_PLATFORM_ERROR,
+                            "X11: RandR gamma ramp support seems broken");
+            _glfw.x11.randr.gammaBroken = GL_TRUE;
+        }
+
+        XRRFreeScreenResources(sr);
     }
 
     if (XQueryExtension(_glfw.x11.display,
@@ -701,7 +718,6 @@ int _glfwPlatformInit(void)
 
     _glfwInitTimer();
     _glfwInitJoysticks();
-    _glfwInitGammaRamp();
 
     return GL_TRUE;
 }