Re-implement more efficient GetBust

 #100 Was lost in a merge at a2049be03ac5adf025f645815c70d8d9b1316425

before:
````
BenchmarkGetHead         5000000               613 ns/op
BenchmarkGetHelm          200000              9464 ns/op
BenchmarkGetBody          100000             20008 ns/op
BenchmarkGetBust          100000             26295 ns/op
````

after:
````
BenchmarkGetHead         5000000               623 ns/op
BenchmarkGetHelm          200000              9424 ns/op
BenchmarkGetBody          100000             19115 ns/op
BenchmarkGetBust          200000             14813 ns/op
````

Based on the benchmarks, this seems like an expensive operation. Perhaps you could just do something like...

```go
skin.Processed.Pix = append(skin.Process.Pix, make([]uint8, (LaWidth+TorsoWidth+RaWidth) * 4 * LlHeight)))
```

to increase the existing image size, then just fastDraw onto that.