Add a vendored copy of SDL

vendored/sdl/test/relative_mode.markdown

@@ -0,0 +1,58 @@
+Relative mode testing
+=====================
+
+See test program at the bottom of this file.
+
+Initial tests:
+
+ - When in relative mode, the mouse shouldn't be moveable outside of the window.
+ - When the cursor is outside the window when relative mode is enabled, mouse
+   clicks should not go to whatever app was under the cursor previously.
+ - When alt/cmd-tabbing between a relative mode app and another app, clicks when
+   in the relative mode app should also not go to whatever app was under the
+   cursor previously.
+
+
+Code
+====
+
+    #include <SDL.h>
+
+    int PollEvents()
+    {
+        SDL_Event event;
+        while (SDL_PollEvent(&event))
+        {
+            switch (event.type)
+            {
+                case SDL_QUIT:
+                    return 1;
+                default:
+                    break;
+            }
+        }
+
+        return 0;
+    }
+
+    int main(int argc, char *argv[])
+    {
+        SDL_Window *win;
+
+        SDL_Init(SDL_INIT_VIDEO);
+
+        win = SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, 0);
+        SDL_SetRelativeMouseMode(SDL_TRUE);
+
+        while (1)
+        {
+            if (PollEvents())
+                break;
+        }
+
+        SDL_DestroyWindow(win);
+
+        SDL_Quit();
+
+        return 0;
+    }