function GetTempDir: String;
Returns fully qualified path of the temporary directory, with trailing backslash. This does not use the Win32 function GetTempPath, due to platform differences.
Gets the temporary file path as follows:
1. The path specified by the TMP environment variable.
2. The path specified by the TEMP environment variable, if TMP is not defined or if TMP specifies a directory that does not exist.
3. The Windows directory, if both TMP and TEMP are not defined or specify nonexistent directories.