uterm: video: change blit'ting logic

uterm: video: change blit'ting logic

Instead of passing width/height separately, we now use the values from the
buffer object. The caller has to manipulate the buffer object to change
these values. In fact, they can simply create a buffer object on the stack
with the same values copied from the real buffer and just adjust the
offset, width, height and stride. With the help of the stride value, it is
possible to even shrink the buffer horizontally.

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