Idempotent changes to PixelAttribute class (preparing for next patch)
Add a --drawalpha mode that keeps water (etc.) transparent to great depths

This new mode averages the colors of all transparent blocks, instead of
making the colors progressively darker and more opaque.

This 'average' mode is now the default when using --drawalpha. It can be
explicitly selected using --drawalpha=average. The old modes can be selected
using --drawalpha=cumulative[-darken].

It is recommended to change the colors of water as well. These are in a
separate patch.
Improve the colors of water (esp. for --drawalpha) & make lava transparent.

As the best color for water depends on the rendering parameters
(i.e. the --drawalpha mode: none, average or cumulative), two
additional colors files are provided for these alternatives.