Fix SVGPlugin, adapt to lunasvg change of default bitmap format

[NaLiJa]

In the demo the colors of the svg are wrong, because lunasvg has changed the default bitmap format to argb:
sammycage/lunasvg#73

in the appveyor.yml it pulls version 2.3.1 of lunasvg, so this must be updated

[mikke89]

Thanks for another pull request!

This one in particular is already fixed in the 'effects' branch: 70bcdc7

The effects branch also makes it correctly rendered in terms of pre-multiplied alpha. I think it's better to wait for that branch to be merged, rather than dealing with the conflicts later. So we just needs a bit more patience in this case :) Cheers, keep the contributions coming though, they are very much appreciated.