It appears that trying to use GETPIXEL with a screen always returns 0 :
CREATESCREEN 1,0,640,480
USESCREEN 1
DRAWLINE 0,0,100,0,RGB(255,255,0)
DRAWLINE 0,1,100,0,RGB(255,255,0)
DEBUG GETPIXEL(1,0)
USESCREEN -1
Fixed in next update. The bitmap size was ignored and the internal texture size was used instead. Use a power of 2 screen to get rid of that so far.