[Proper matte for PNG8/GIF export]

When I export a PNG8 or GIF with matte, what I would expect (based on other software) is that parts of the image with an alpha of 0 would be transparent, and those with alpha above 0 would have the matte.

The actual export behaviour is that matte is applied everywhere on the image. This means that it is hard to use an image made this way on the web, because if the web browser renders the background colour just slightly differently to the image matte, a "box" can be seen around the image.