[FIX] Report: compatibility issues on Windows

This commit fixes two different issues specific to Windows:
* NamedTemporaryFile behavior, requiring a file to be closed before being
  accessed a second time
* Wkhtmltopdf crash when the document's dpi is inferor to 96
[FIX] report: windows: lower level of file opening/closing, which to find wkhtmltpdf

* Use of 'which' to find the wkhtmltopdf binary (allow the win32 service to find it)
* Use of mkstemp and manual close of the file descriptors