vcl: SalGraphics::mirror() - always use GetGraphicsWidth()

There's never a case where we need to get the width from VirtualDevice,
we really should only get it from the SalGraphics instance. This vastly
simplifies matters.

Change-Id: I8bae9c163fcb0ce5d064d802c2783437124ebcb2
Reviewed-on: https://gerrit.libreoffice.org/12473
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>