vectorization quiz

[Francesco Potortì]

I thought a little, but I do not see the way of vectorising this.  It's
a loop that is run only few times, but is called very frequently.
Essentially, there are few 2-D maps stacked one over the other which
must be shifted each by a variable amount and then summed to obtain a
new 2-D map.  Here is the code:

    for mi = 1:m
      llka += llkm(by+offm(mi,1), bx+offm(mi,2), mi);
    endfor

where:
m          ==>     5
size(llka) ==>     156 222
size(llkm) ==>     186 252 5
by         ==>     16:172
bx         ==>     16:238
size(offm) ==>     5 2
offm(m,i)  ==>     a scalar in the range [-15;15]

Thanks in advance to those wishing to try :)

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