Avoid negative text size in P.3.0

I wrapped the difference in a call to abs() to avoid ending up with
a negative text size when the mouse is in the left part of the screen.

The behavior of textSize() with negative values in unspecified;
starting the sketch with the mouse in the left part of the canvas,
I observed first a downward letter A, scaled according to the cursor
horizontal position, then flipped in upright position when the mouse
crosses the middle of the canvas; from this point, whenever the cursor
is in the left part of the canvas, the letter's height is fixed to
a small value, which no longer changes according to horizontal position.