...and move it into the only .cxx that uses it; making just enough of
BitmapColor constexpr as is useful here

Change-Id: I8a035f67f4b218dcef2f100814c62b25fb32d5be
Reviewed-on: https://gerrit.libreoffice.org/47750Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>