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.

cmake/platforms/emscripten.cmake

@@ -30,6 +30,7 @@ list(TRANSFORM emcc_export_list PREPEND \")
 list(TRANSFORM emcc_export_list APPEND \")
 string(JOIN "," emcc_export_string ${emcc_export_list})
 set(CMAKE_C_LINK_FLAGS "\
+-s ALLOW_MEMORY_GROWTH=1 \
 -s EXPORTED_FUNCTIONS='[${emcc_export_string}]' \
 -s EXTRA_EXPORTED_RUNTIME_METHODS='[cwrap,callMain]'")