Add and revise function comments in surface.lua
Tweak introductory comment in light.lua
Add comment for tracking documentation to transform.lua
Document perlin noise textures in texture.lua

Also tweak the comment for texture.fourier_series, add
documentation-tracking comments, and do some other documentation
tweaks.
configure.ac: Configure awk
Add process-txtin.awk

This is a simple text-processor for creating documentation files, and
can automatically generate function documentation from Lua function
comments.