fix: tint and spine skeleton/slot/attachment color

[davidetan]

This PR should fix two issues about colors:

• Fix the tint issue reported here: https://github.com/pixijs/spine-v8/issues/18
• Fix the final attachment color. The attachment color is the result of the skeleton, slot and attachment color. Previously only the slot color was used. You can see this from the white part in the hair of the girl in this example. Hair is colored using the attachment color, not the slot color.