Sticker decal mode & decal improvements

These changes add a Sticker decal mode and improve the rendering of the other decal modes.

In Sticker mode the decal is applied once. The area around the decal and any transparent areas in the decal are rendered with the Paint color.

In the other decal modes the paint color is shown through any transparent portions of the image, as you would expect.

![stickers](https://f.cloud.github.com/assets/1305026/50334/0c8c11dc-59a0-11e2-8f46-99157af3fdb7.png)