GETPOINTS: Get points from mouse clicks
[p,t] = getpoints(siz)
siz: size of the capture area. Can be either
a) a scalar: then a centered squared area is taken [-siz siz -siz siz]
b) a 2 elem vec: then a centered rectangular area is taken [-siz(1) siz(1) -siz(2) siz(2)]
c) a 4 elem vec: then an arbitrary rectangular area is taken [siz(1) siz(2) siz(3) siz(4)]
p: Matrix (2xQp) with points generated with the left button
t: Matrix (2xQt) with points generated with the right button