Remove blt demo.

Support was pretty buggy and maintaining it was work.
scripts: tolerate being called from other dirs.

In particular, calling them as e.g. `../flash.sh` from the build/ dir is
terribly convenient.
BUILD.conf: generate debug symbols.

Almost exactly a year into this project, I found a need for a debugger.
Update for revised resource management model.

Gone are the static rasterizer instances.  The Arena reigns supreme.

Demos allocate their internal state from the Arena; the constructors
handle initialization.  The run functions still orchestrate everything,
but are showing a bit more regular of a pattern, such that I might be
able to extract it.
demo/tunnel: build both tablegen versions.

This should help keep the code from rotting.

I've used the online version of the tunnel code in the demo reel simply
because it drops the reel binary's size under 64KiB, which seems like as
good a reason as any.
demo/xor_pattern: respect user button.