Add DecodeDraw to allow custom image types

[diamondburned]

Originally, the library only has a Decode function that would always return an *image.NRGBA. This is a minor inconvenience, as some other functions (such as libjpeg's) can only do RGBA or other image types.

As a fix for the above inconvenience, a DecodeDraw function was added:

func DecodeDraw(dst draw.Image, hash string, punch float64) error

This function decodes the given hash and punch (which is now float64 for granularity) and draws the decoded image onto the given draw.Image.

The older Decode function now calls DecodeDraw internally on an *image.NRGBA. This ensures that the API isn't broken.

Benchmarks

After the changes, with Decode now calling DecodeDraw:

goos: linux
goarch: amd64
pkg: github.com/bbrks/go-blurhash
BenchmarkComponents/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8          62727241            18.2 ns/op
BenchmarkComponents/LNAdApj[00aymkj[TKay9}ay-Sj[-8          66509407            17.9 ns/op
BenchmarkComponents/LNMF%n00%#MwS|WCWEM{R*bbWBbH-8          56069827            18.0 ns/op
BenchmarkComponents/KJG8_@Dgx]_4V?xuyE%NRj-8                67373836            17.9 ns/op
BenchmarkDecode/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8                  1254        924818 ns/op
BenchmarkDecode/LNAdApj[00aymkj[TKay9}ay-Sj[-8                  1296        882316 ns/op
BenchmarkDecode/LNMF%n00%#MwS|WCWEM{R*bbWBbH-8                  1214        940441 ns/op
BenchmarkDecode/KJG8_@Dgx]_4V?xuyE%NRj-8                        1431        805385 ns/op
BenchmarkEncode/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8                     6     187144765 ns/op
BenchmarkEncode/LNAdApj[00aymkj[TKay9}ay-Sj[-8                     9     122129504 ns/op
PASS
ok      github.com/bbrks/go-blurhash    16.277s

Before the changes:

goos: linux
goarch: amd64
pkg: github.com/bbrks/go-blurhash
BenchmarkComponents/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8          67750879            18.4 ns/op
BenchmarkComponents/LNAdApj[00aymkj[TKay9}ay-Sj[-8          56951818            23.9 ns/op
BenchmarkComponents/LNMF%n00%#MwS|WCWEM{R*bbWBbH-8          41726830            26.1 ns/op
BenchmarkComponents/KJG8_@Dgx]_4V?xuyE%NRj-8                50471245            26.8 ns/op
BenchmarkDecode/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8                   962       1295024 ns/op
BenchmarkDecode/LNAdApj[00aymkj[TKay9}ay-Sj[-8                  1233        976301 ns/op
BenchmarkDecode/LNMF%n00%#MwS|WCWEM{R*bbWBbH-8                  1249       1128537 ns/op
BenchmarkDecode/KJG8_@Dgx]_4V?xuyE%NRj-8                        1171       1109666 ns/op
BenchmarkEncode/LFE.@D9F01_2%L%MIVD*9Goe-;WB-8                     6     190812766 ns/op
BenchmarkEncode/LNAdApj[00aymkj[TKay9}ay-Sj[-8                     9     120429582 ns/op
PASS
ok      github.com/bbrks/go-blurhash    16.912s

Note: the performance is roughly the same, as the compiler is probably smart enough to know and inline the function call.

[diamondburned]

Oh oops, it seems like I've accidentally opened an issue and not a pull request.

[diamondburned]

PR opened.