Speedup warm()

- Fill "out" image with transparent pixels in one operation.
- Calculate "percent" after we determine a pixel should be modified.

Before:
```
$ go test -bench=BenchmarkWarming -benchtime=20s
PASS
BenchmarkWarming	       5	5019502926 ns/op
ok  	github.com/dustin/go-heatmap	35.382s
```

After:
```
$ go test -bench=BenchmarkWarming -benchtime=20s
PASS
BenchmarkWarming	      10	3464537436 ns/op
ok  	github.com/dustin/go-heatmap	44.146s
```