Increase the height of the default face in the current buffer by INC steps.
If the new height is other than the default, `text-scale-mode' is enabled.
Each step scales the height of the default face by the variable
`text-scale-mode-step' (a negative number of steps decreases the
height by the same amount). As a special case, an argument of 0
will remove any scaling currently active.
(fn INC)