USAGE: result=dodriftcorrect(el [, x, y, draw, allowsetup])
el: eyelink default values
x,y: position of driftcorrection target
draw: set to 1 to draw driftcorrection target
allowsetup: set to 1 to allow to go in to go to trackersetup
Psychtoolbox/PsychHardware/EyelinkToolbox/EyelinkBasic/EyelinkDoDriftCorrect.m