vcl: remove ImplImageBmp as friend of OutputDevice

Change-Id: Ia212aaa10840bae30508579e17ed8d2e9781c46d
Reviewed-on: https://gerrit.libreoffice.org/13035
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>