win32: create screenshots path if needed. Fall back to Pictures folder

Ask Windows to create the Screenshots folder if it doesn't exist.
Fall back to Pictures folder if the Screenshots folder can't be
determined.
Issue #1761

core/oslib/oslib.cpp

@@ -244,7 +244,8 @@ void saveScreenshot(const std::string& name, const std::vector<u8>& data)
 static std::string getScreenshotsPath()
 {
 	wchar_t *screenshotPath;
-	if (FAILED(SHGetKnownFolderPath(FOLDERID_Screenshots, KF_FLAG_DEFAULT, NULL, &screenshotPath)))
+	if (FAILED(SHGetKnownFolderPath(FOLDERID_Screenshots, KF_FLAG_CREATE, NULL, &screenshotPath))
+			&& FAILED(SHGetKnownFolderPath(FOLDERID_Pictures, KF_FLAG_CREATE, NULL, &screenshotPath)))
 		return get_writable_config_path("");
 	nowide::stackstring path;
 	std::string ret;