Support for RGBA images

[rfuest]

At the moment the alpha channel of RGBA images is simply ignored. When e-g gains support for alpha channel the appropriate color type should be used.

Until then RGBA images should either not be supported at all or the output should assume a black background color. To correctly display images with a black background the color values would need to be multiplied with the alpha channel (QOI uses un-premultiplied colors).