Basic implementation of the paper "Improved Alpha-Tested Magnification for
Vector Textures and Special Effects" by Chris Green, Valve.  
http://www.valvesoftware.com/publications/2007/SIGGRAPH2007_AlphaTestedMagnification.pdf

Run genfield.py over a large monochrome image (at least 1024x1024) to generate
the compressed distance field image.  e.g.

    python genfield.py --size=64 py.png

Produces py.df.png (or use -o to specify output file).

Then preview the alpha-tested rendering using renderfield.py

    python renderfield.py py.df.png

Following keys modify the shader operation:

    A   Toggle antialiasing
    B   Toggle bidirectional (see below)
    O   Toggle outline
    G   Toggle glow
    ,   Decrease outline/glow
    .   Increase outline/glow

Bidirectional fields (see last page of Valve paper) can be generated by
passing -b to genfield.py, and enabled in renderfield.py by pressing 'B'.
Bidirectional field rendering requires shader, and seems to look worse in
every case.  Glow, outline and AA have not (yet) been implemented for
bidirectional.
