vt(4): Add vd_bitblt_bmp_t callback

The code was already there in all backends, we just expose it. This is
used to display the splash screen.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270431 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f