terraingenerator: output image contains copies of the entire input image #3528

Describe the bug When using the terraingenerator tool, instead of each tile being painted into the new image, an entire copy of the original tileset image is pasted.

(Suspect it was broken by this change: https://github.com/mapeditor/tiled/commit/7f7589e54d824fc28949f5ad914a8e5b2377f7cd#diff-c944b3b1ed7da5b5abd516d87830bafe51b96c7491a490e3a9093c5e025b8c2d )

To Reproduce Steps to reproduce the behavior:

1. Create a tileset using wangtiles for the "base terrains"
2. Use tiled.terraingenerator to build a complete terrain set
3. Look at the output image

Expected behavior Each tile combination is added to the image, instead of the entire source terrain

Media Input terrain image:

Output image:

Specifications:

• OS: Ubuntu 22.04
• Tiled Version: 1.9.2