Disable local terrain generation in online mode

[satoshinm]

for #188 Use only terrain from server, remove local generation when online

[satoshinm]

Disabling local terrain generation when the client is enabled is easy enough:

but this breaks server.py / native client

[satoshinm]

Native build of client connecting to server.py, shows only blocks which the user added beyond the terrain:

except for the outer edges which do show the terrain locally which I'm not sure why

[satoshinm]

Not ideal, but #ifdefing __EMSCRIPTEN__ to disable local terrain generation when online, when running under emscripten since it is connecting to WebSandboxMC which sends the full terrain, and enabling it when in the native build since server.py sends only a delta, is the most expedient.