Make Navi work with love 0.9.0+

love.graphics.setLine was removed in 0.9.0 according to
https://love2d.org/wiki/love.graphics.setLine
Make text scrolling work even in the presence of transformations

love.graphics.setScissor isn't affected by graphical transformations
like translate and scale. Since text scrolling used this function, it
would not work properly when drawing Navi messages when the graphical
context had transformation applied.

love.graphics.setStencil fills a similar purpose, but is affected by
graphical transformations.