Source map paths behave badly on Win32

[ghost]

Some example bad outputs in the source map for Windows:
../C:\foo\bar\baz.scss
../../../../../C:\foo\bar\baz.scss
../C:\foo\bar/qux.scss
C:\foo\bar/qux/quux.scss

When source_map_file and input_path are in the same directory, it does not output relative paths in the source map and instead uses mixed slashes as shown above.

Workaround: replace backslashes with forward slashes in both input_path and source_map_file.

I'd be OK with it using forward slashes as ultimately this will be served from a URL and forward slashes are appropriate for URLs, but it doesn't make the URL relative and that's an issue.

[am11]

Possible dup of #242. Even the JSON in sourcemap is invalid because \ is not escaped.

[HamptonMakes]

This seems to be fixed over at #242