text: add bbulk renderer

text: add bbulk renderer

The bbulk renderer is very similar to the bblit renderer but it assembles
a request-buffer of all characters and then pushes these requests via a
vector-call to the video hardware.
This turns out to increase performance slightly as we do not call into the
video subsystem for every characters but only once.

This renderer can reduce performance when used with partial-redraws (which
are not implemented, yet), so we keep the bblit renderer around.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>