Fix crash when saving tilemap's cel image

[martincapello]

After applying this PR:

1. A script can save a cel's image from a tilemap layer without crashing Aseprite.
2. The saved image is the rendered image using the tileset tiles for each tilemap index.
3. A new lua test is added.

Fix #4069

[dacap]

Didn't review this, waiting for @Gasparoken, anyway it would be nice to add a note about any new sprite added in the tests/sprites/README.md file.

[Gasparoken]

I'll review this today. Sorry for the delay.

[Gasparoken]

Works great! I think the name MakeStdTilemap could be improved to describe:
"Make a blank sprite with Tilemap layer and specific Tileset"

[martincapello]

Would MakeStdTilemapSpriteWithTileset work?

[Gasparoken]

Would MakeStdTilemapSpriteWithTileset work?

LGTM!😊

[Gasparoken]

I was testing and found that this solution saves an image with the size of the sprite (which could be different than the size of the image in the cel) and the image is placed at origin = 0.0. Therefore, it is necessary to shrink the image before saving, or define the correct size of the final image from the beginning.

[Gasparoken]

LGTM! 🙌