[libpc] Perf update

[Michael P. Gerlek]

I was testing pcview today on the MtStHelens data set, using a decimation
filter to chop it down to 1M points and a colorizer filter to make the
points look pretty.

Informally, it looks like with the native libpc driver it does the point
reading/processing in about 1.0 seconds, and with the libLAS driver it takes
about 2.5 seconds.  Pretty sweet!

Also, for this test anyway, pcview is a bit faster than Martin's lasview.

-mpg