Return random variable(s) with linear distribution within [0,1] and 0 outside; m specifies the slope

In other word
P(x)= mx+b if 0<=x<=1
P(x)= 0 otherwise

where b=1-m/2 as int P(x) should be 1

Usage: ran=randl(m,SIZE)

E.g. X=randl(1,[20,20])
X is matrix with size [20,20] and with distribution
p(x)=x+0.5 if 0<=x<=1 and p(x)=0 otherwise