GetBust Efficiency

As opposed to just cropping the GetBody, by manually rendering again we boost performance by quite a hefty degree (also a large improvement with the imaging.Crop).

With the original crop function (as used in the PR)
````
BenchmarkGetHead          500000              4829 ns/op
BenchmarkGetHelm          200000              7190 ns/op
BenchmarkGetBody           50000             30426 ns/op
BenchmarkGetBustOld           50000             45033 ns/op
BenchmarkGetBustNew        100000             18983 ns/op
````

With the imaging.Crop (as used in #97)
````
BenchmarkGetHead         5000000               637 ns/op
BenchmarkGetHelm         1000000              2829 ns/op
BenchmarkGetBody          200000             12230 ns/op
BenchmarkGetBustOld       100000             17920 ns/op
BenchmarkGetBustNew          200000              8227 ns/op
````

Unfortunately it results in slightly duplicated code (GetBust vs. GetBody). I expect there is a way to make it neater again if it's a big deal (might need some guidance though).