Tileset export feature

[B-Reif]

When working with tile data, it can be useful to export an image of the Tileset in order to render each tile on the texture exactly once.

The Aseprite beta UI doesn't currently support exporting a Tileset image directly. For now, @dacap shared a lua script that will do the same thing. The function in this PR is tested against an image generated by this lua script.

[B-Reif]

I'm not sure how we want to go about resolving the indexed pixels in this case. This may change depending on how we want to factor the pixel format data, per #4. We could store a reference to the required index-resolution data.

EDIT: We could do something similar to Frame<'a> where we expose a type that has a reference back to the file itself.

[B-Reif]

Ok, this PR makes a few new changes:

• Indexed pixels are now resolved during .image() instead of during parsing. The pixel indices are still validated to be in the palette's range ahead of time.
• Pixels now has a method for converting to image::Rgba. It takes a closure for resolving Indexed pixels.
• Grayscale now fully supported with tests, removed assertions.
• Also, exporting a Tileset image is now invoked on the File by passing in the Tileset's id.

[alpine-alpaca]

Do you think this is ready for review/merge?

[B-Reif]

Do you think this is ready for review/merge?

Yes, I think so. We might want to merge both this and #6 together as the 0.3 release, or we can increment the version twice.

[B-Reif]

@alpine-alpaca Are there any other changes you'd like here?

[alpine-alpaca]

This is included in the merged PR, right?

[alpine-alpaca]

Handled by #9