fix: Use resolved path when loading single-image tilesets (#3826)

Fixes an inconsistency in `TiledAtlas` path resolution between
single-image and multi-image tilesets when using external TSX files with
relative image paths. When loading a `RenderableTiledMap` with a single
image from an external TSX tileset that references images using relative
paths, the image loading would fail. The single-image code path was
using the atlas key (original relative path like `../images/name.png`)
to load the image, while the multi-image code path correctly used the
resolved path (tileset directory + relative path like
`tiles/../images/name.png`).

This inconsistency meant that maps with one image would fail to load in
scenarios where maps with multiple images would succeed.

Updated the single-image path to:
- Use `mappedEntry.$1` (resolved image source) for loading images,
matching the multi-image behavior
- Use `tiledImage.source!` for offset map keys, maintaining consistency
with how the multi-image path stores offsets

This ensures both code paths follow the same path resolution logic
applied in `mappedImageList`.