Fix FontForge script generation on Windows

[Rirush]

On Windows, the script contains paths that look like this:

Import("a\b\c", 0)

FontForge interprets this path as abc instead of a/b/c. This patch escapes backslashes on Windows, generating paths like this:

Import("a\\b\\c", 0)

These paths are correctly treated as a/b/c, making this work on Windows.