Set ALLOW_MEMORY_GROWTH in the Emscripten build.

Set ALLOW_MEMORY_GROWTH in the Emscripten build.

How embarrassing. When I updated the Emscripten build to use WASM, a
major reason I bothered to do it at all was that I'd heard that WASM
was capable of reallocating its memory arena larger on the fly. Turns
out that it _can_, but only if you specifically set the option in
Emscripten to allow it.

With this option set, I can finish a 25x25 Galaxies, where previously
the game would crash part way through (and not even a very large part)
with errors about memory growth in the Javascript console.