Improved tearing test.

Added support for arbitrary swap intervals, including negative ones if
(GLX|WGL)_EXT_swap_control_tear is available.

tests/tearing.c

@@ -36,6 +36,7 @@
 
 #include "getopt.h"
 
+static GLboolean swap_tear;
 static int swap_interval;
 static double frame_rate;
 
@@ -51,8 +52,10 @@ static void update_window_title(GLFWwindow* window)
 {
     char title[256];
 
-    sprintf(title, "Tearing detector (interval %i, %0.1f Hz)",
-            swap_interval, frame_rate);
+    sprintf(title, "Tearing detector (interval %i%s, %0.1f Hz)",
+            swap_interval,
+            (swap_tear && swap_interval < 0) ? " (swap tear)" : "",
+            frame_rate);
 
     glfwSetWindowTitle(window, title);
 }
@@ -81,9 +84,28 @@ static void key_callback(GLFWwindow* window, int key, int scancode, int action,
 
     switch (key)
     {
-        case GLFW_KEY_SPACE:
-            set_swap_interval(window, 1 - swap_interval);
+        case GLFW_KEY_UP:
+        {
+            if (swap_interval + 1 > swap_interval)
+                set_swap_interval(window, swap_interval + 1);
             break;
+        }
+
+        case GLFW_KEY_DOWN:
+        {
+            if (swap_tear)
+            {
+                if (swap_interval - 1 < swap_interval)
+                    set_swap_interval(window, swap_interval - 1);
+            }
+            else
+            {
+                if (swap_interval - 1 >= 0)
+                    set_swap_interval(window, swap_interval - 1);
+            }
+            break;
+        }
+
         case GLFW_KEY_ESCAPE:
             glfwSetWindowShouldClose(window, 1);
             break;
@@ -152,6 +174,8 @@ int main(int argc, char** argv)
 
     last_time = glfwGetTime();
     frame_rate = 0.0;
+    swap_tear = (glfwExtensionSupported("WGL_EXT_swap_control_tear") ||
+                 glfwExtensionSupported("GLX_EXT_swap_control_tear"));
 
     glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
     glfwSetKeyCallback(window, key_callback);