Faster drawing function

Go's draw function has to take into account alpha transparency and blending when drawing images, but Minecraft skins don't have partially transparent components. With this knowledge, we can create a simpler, faster function, with a speed increase up to 5x in some cases.

```
Before:
BenchmarkSetup	2000000000	         0.00 ns/op
BenchmarkGetHead	 2000000	       843 ns/op
BenchmarkGetHelm	  200000	     13064 ns/op
BenchmarkGetBody	  100000	     25182 ns/op
BenchmarkGetBust	   50000	     32185 ns/op

After:
BenchmarkSetup	2000000000	         0.00 ns/op
BenchmarkGetHead	 2000000	       851 ns/op
BenchmarkGetHelm	 1000000	      2300 ns/op
BenchmarkGetBody	  200000	     12920 ns/op
BenchmarkGetBust	  100000	     14828 ns/op
```

And just for fun, when the benchmarks were first added two days ago :smile: 

```
BenchmarkGetHead	  500000	      6638 ns/op
BenchmarkGetHelm	  200000	      9264 ns/op
BenchmarkGetBody	   50000	     38927 ns/op
```