Use a $PWD independent method to determine the location of the data directory

[pwaller]

For testing purposes it will be desirable to run for example:

go run ${GOPATH}/src/github.com/go-gl/examples/nehe08/*.go

.. without being concerned about the run directory. Currently, I get [e] Failed to load texture: ../data/Glass.tga.

This package provides what looks to me to be a reasonable way of figuring out package-relative paths - I use it in glh to determine a font path.