demo/tunnel: modernize types, enable msigs.

The type fix is more than just spelling -- it postpones the eventual
uxtb instruction to as late as possible, saving a cycle in the shading
critical path.
demo/rook: switch vertex data to half-precision.

This cuts the vertex table size by 50%, shaving off just over 10KiB.
demo/tunnel: optimize inner loop.

The previous version was written for clarity rather than performance,
and did a lot of unnecessary work -- looking up the same table entry
four times, for example, once for each quadrant.

I've rewritten it as a loop over a single quadrant, generating four
pixels per iteration.  Conveniently, this recognizes that diagonally
opposed quadrants are identical and reuses the shader output, cutting
the cost of shader effects -- so I've added some.