PDF: Use / as path separators in latex input only

Fixes compile error on Windows for 5040f3e
Reverted back to canonical file separators </> in all places except for
arguments to the LaTeX builder and in TEXINPUTS

See #1151.

Note: Temporary directories still fail to be removed in Windows due to
call of ByteString.Lazy.readFile creating process ownership of the
compiled pdf file.