Yes I know I can insert this in the bitmap but... if the outline/shadow is bigger the distance between letters is bigger too.
The font definition can contain a parameter called EXPAND_WIDTH, which is a number of pixels to be added to every letter's with. It can be a negative number, so you can say, e.g.
EXPAND_WIDTH = -1
to make all the letters 1 pixel thinner.
And the trick using layers works only with 1 pixel outlines.
That's not necessarily true. You can achieve thicker outlines the same way, but you'll need to use more layers.
