RGBA vs NRGBA

The webp libraries return non-premultiplied alpha values. This library wraps those values in RGBA structs which represent premultiplied values.

Everywhere `RGBA{}` is created with data returned by the webp decoder, it should be `NRGBA{}`.

e.g. this fixes one of them: https://github.com/chai2010/webp/compare/master...jamesshoebox:nrgba?expand=1

Here's a (very) simple program that illustrates the problem. If you pass in a webp with alpha, you'll see that it doesn't blend correctly with the white background.
https://gist.github.com/jamesshoebox/30ca5beada606842f98328481bf17f8d

Search the decoder code for every RGBA{} constructed from values returned by the WebP libraries, using the issue's alpha-blending example to reproduce the problem first. Replace only those mismatched representations with NRGBA{} and verify that a WebP image with alpha blends correctly against a white background.

Written by the indexing model from the issue text.