[libpc] quick perf look

[Michael P. Gerlek]

I ran "pc2pc -i MtStHelens.las -o x.las" against "las2las -i MtStHelens.las
-o x.las" this afternoon.  I was getting about 3.0 secs for pc2pc and 2.5
secs for las2las.  [The writer still isn't bit for bit right in the headers,
but it's close enough for this exercise.]

[Yes, this is using the "LiblasReader/Writer" path, so we're really just
looking at overhead here.  If we used the native LasReader/Writer, I expect
us to be in much better shape.]

3.0-vs-2.5 isn't as good as I'd like, but it's not unreasonable.  Looking at
the profiles for pc2pc, functions in the libpc:: namespace seem to account
for ~15-20% of the runtime, but I'm not real confident of the amount of
sampling for such a short run.

A big part of that, however, does seem to be the code that moves the
individual fields between the liblas::Point and the libpc::PointData -- on
both the read and write sides, the code to move the fields is a long,
branchless sequence of calls to Point.SetFoo() and Point.GetFoo().  If I
could use Point's direct SetData/GetData calls to map into the PointData
buffer, a lot of that might go away.

So anyway, I'm not too worried at this point.

-mpg