MFC fbd(4) and vt_fb disentanglement:

MFC fbd(4) and vt_fb disentanglement:

r268472 (ray):

  Should check fb_read method presence instead of double check for fb_write.

r269620 (nwhitehorn):

  Retire various intertwined bits of fbd(4) and vt_fb, in particular the
  pixel modification indirection. No actual drivers use it and those
  that might (e.g. creatorfb) use custom implementations of
  vd_bitbltchr().

Relnotes:	No
Sponsored by:	The FreeBSD Foundation