convert SV_BMP_ constants to enum

Change-Id: I46cb7f649fa44d8967e0e7da03583157d2706f05
Reviewed-on: https://gerrit.libreoffice.org/12018
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>