fdo#74702 Move GetBitCount() and GetAlphaBitCount into correct classes

GetBitCount() works differently for VirtualDevices. GetAlphaBitCount()
is really only used by VirtualDevice, so moved functionality from
OutputDevice to VirtualDevice.

Change-Id: Ic00e32f1fa385542bcce8c9475f0ea5eb9a077f9
Reviewed-on: https://gerrit.libreoffice.org/8722
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>