Render the pager's continuation prompt with whatever colors are
already active instead of forcing the console defaults.

While here, add a TODO to document the existing set_color
None-value mismatch with actual behavior, as I spotted this
while fixing the pager issue.

Limit text scrolling to the rows that actually belong to the character
grid so partial bottom margins cannot leak into the last line.

This fixes an oddity I had been observing for a while where the last
line was only half-colored.

Honor the raster scrolling contract by repainting the newly exposed
portion of the source area after shifting the canvas contents upward.

This is related to the previous commit and ensures that new lines are
drawn with the correct background color, and that running CLS causes
the whole screen to be "blanked" with the new background color.