Center the keyboard into the string and make it independent of the
screen size
For obtaining an hexagonal matrix the keys are superposed and the
outsides of the hexagon are transparent.
When a key is pressed, the system draws the whole image, overwriting
part of the neighbors keys with the background because the outsides are
transparent.

The solution is capture an snapshot of the keyboard before any key is
pressed and use it as background.

Other solution would be paint the outside of the hexagon with the same
color of the hexagon, but this produces a bad effect