vt: Add vd_bitblt_argb

This blit an ARGB image on the dedicated vd.
This also adds vt_fb_bitblt_argb which will works for most of the vt backends

Differential Revision: https://reviews.freebsd.org/D45929
Reviewed by: tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG