video: Avoid starting a new line to close to the bottom

video: Avoid starting a new line to close to the bottom

When starting a new text line, an assumption is made that the current
vertical position is a multiple of the character height. When this is
not true, characters can be written after the end of the framebuffer.

This can causes crashes and strange errors from QEMU.

Adjust the scrolling check when processing a newline character, to
avoid any problems.

Add some comments to make things a little clearer.

Signed-off-by: Simon Glass <sjg@chromium.org>