### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

return true;
				} else if (components[2] == "color") {
					ERR_FAIL_COND_V(p_value.get_type() != Variant::COLOR, false);
					while (terrain_set_index >= terrain_sets.size()) {
						add_terrain_set();
					}
					while (terrain_index >= terrain_sets[terrain_set_index].terrains.size()) {
						add_terrain(terrain_set_index);
					}
					set_terrain_color(terrain_set_index, terrain_index, p_value);
					return true;
				}
			}
		} else if (components.size() == 2 && components[0].begins_with("navigation_layer_") && components[0].trim_prefix("navigation_layer_").is_valid_int()) {
			
			int index = components[0].trim_prefix("navigation_layer_").to_int();
			ERR_FAIL_COND_V(index < 0, false);
			if (components[1] == "layers") {
				ERR_FAIL_COND_V(p_value.get_type() != Variant::INT, false);
				while (index >= navigation_layers.size()) {
					add_navigation_layer();
				}
				set_navigation_layer_layers(index, p_value);
				return true;
			}
		} else if (components.size() == 2 && components[0].begins_with("custom_data_layer_") && components[0].trim_prefix("custom_data_layer_").is_valid_int()) {
			
			int index = components[0].trim_prefix("custom_data_layer_").to_int();
			ERR_FAIL_COND_V(index < 0, false);
			if (components[1] == "name") {