tone: Over 100% volume when using Attack parameter
I first noticed this issue in my game that plays a sound when firing, when the player fires rapidly. I've recreated the problem here in a much smaller test cart.

Starting with `w4 create --zig test`, and editing `main.zig` to be:

```
const w4 = @import("wasm4.zig");

var frame: u64 = 0;
export fn update() void {
    if (frame % 15 == 0)
        w4.tone(370, (16 << 24) | 38, 100, w4.TONE_TRIANGLE);
    frame += 1;
}
```

```
$ w4 -V
2.1.0
$ w4 run zig-out/lib/cart.wasm
```

After a second or two the sound starts playing *much* louder.

The problem goes away if I change the second parameter to just '38' (i.e. drop the Attack part of the ADSR envelope).

Here's the cart: 
[cart.wasm.gz](https://github.com/aduros/wasm4/files/7833485/cart.wasm.gz)