SDL_windowsevents.c: fix build against older SDKs.

src/video/windows/SDL_windowsevents.c

@@ -81,6 +81,16 @@
 #define WM_UNICHAR 0x0109
 #endif
 
+#ifndef IS_HIGH_SURROGATE
+#define IS_HIGH_SURROGATE(x)   (((x) >= 0xd800) && ((x) <= 0xdbff))
+#endif
+#ifndef IS_LOW_SURROGATE
+#define IS_LOW_SURROGATE(x)    (((x) >= 0xdc00) && ((x) <= 0xdfff))
+#endif
+#ifndef IS_SURROGATE_PAIR
+#define IS_SURROGATE_PAIR(h,l) (IS_HIGH_SURROGATE(h) && IS_LOW_SURROGATE(l))
+#endif
+
 static SDL_Scancode
 VKeytoScancodeFallback(WPARAM vkey)
 {