Calculates the number of pixels below the baseline that text could descend to when drawn with the Draw Text command.
The y value passed to the 'Draw Text' command represents the baseline of the text being drawn; however the text descends by up to descent pixels below the baseline due to letters like y, g, j, p and q.
If text was to be drawn at a point (x,y), then the box it would occupy could be calculated with: