WebAssembly image dithering example

[Kagamin]

Something like

```wasm.d
extern(C):

static shared size_t len;

const(char)* towastr(string s)
{
     len=s.length;
     return s.ptr;
}

const(char)* echo()
{
     return towastr("Hello from WASM!");
}
```

```wasm.js
...
function wastr(ptr)
{
     var buffer = new Uint8Array(linearMemory.buffer, ptr, 
wasmInstance.exports.len);
     var msg = '';
     for (var i=0; i < buffer.length; i++) {
         msg += String.fromCharCode(buffer[i]);
     }
     return msg;
}
var str=wastr(wasmInstance.exports.echo());
console.log(str);
```