windows: Added WIN_IsWindowsXP, for extreme runtime compatibility checks.

Reference Issue #8666.

src/core/windows/SDL_windows.c

@@ -211,6 +211,15 @@ static BOOL IsWindowsVersionOrGreater(WORD wMajorVersion, WORD wMinorVersion, WO
         return retval;
 #endif
 
+// this is the oldest thing we run on (and we may lose support for this in SDL3 at any time!),
+//  so there's no "OrGreater" as that would always be TRUE. The other functions are here to
+//  ask "can we support a specific feature?" but this function is here to ask "do we need to do
+//  something different for an OS version we probably should abandon?"  :)
+BOOL WIN_IsWindowsXP(void)
+{
+    CHECKWINVER(FALSE, !WIN_IsWindowsVistaOrGreater() && IsWindowsVersionOrGreater(HIBYTE(_WIN32_WINNT_WINXP), LOBYTE(_WIN32_WINNT_WINXP), 0));
+}
+
 BOOL WIN_IsWindowsVistaOrGreater(void)
 {
     CHECKWINVER(TRUE, IsWindowsVersionOrGreater(HIBYTE(_WIN32_WINNT_VISTA), LOBYTE(_WIN32_WINNT_VISTA), 0));

src/core/windows/SDL_windows.h

@@ -145,6 +145,9 @@ extern void WIN_CoUninitialize(void);
 extern HRESULT WIN_RoInitialize(void);
 extern void WIN_RoUninitialize(void);
 
+/* Returns SDL_TRUE if we're running on Windows XP (any service pack). DOES NOT CHECK XP "OR GREATER"! */
+extern BOOL WIN_IsWindowsXP(void);
+
 /* Returns SDL_TRUE if we're running on Windows Vista and newer */
 extern BOOL WIN_IsWindowsVistaOrGreater(void);