Some work improving the viewer led to a couple of changes and some
bugfixes. There's an outstanding bug in the viewer where it doesn't work
with the "empty chunk" implementation that I added, but I'm not sure
why. No huge deal for now.

-Peter Mawhorter
Got the block viewer to work and made it cycle through all available
blocks. Also fiddled with the control scheme so that there's a new
"dive" control used for diving in water (and in air if you can fly).

-Peter Mawhorter
More revision of the static entities and physics, plus some controls
changes. Also got the PAUSED text to show up when alt-tabbing.

-Peter Mawhorter
Switched the entity type for the test world back to a "tester."

-Peter Mawhorter