### 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)

ERR_FAIL_COND_V(p_value.get_type() != Variant::STRING, false);
				while (index >= custom_data_layers.size()) {
					add_custom_data_layer();
				}
				set_custom_data_layer_name(index, p_value);
				return true;
			} else if (components[1] == "type") {
				ERR_FAIL_COND_V(p_value.get_type() != Variant::INT, false);
				while (index >= custom_data_layers.size()) {
					add_custom_data_layer();
				}
				set_custom_data_layer_type(index, Variant::Type(int(p_value)));
				return true;
			}
		} else if (components.size() == 2 && components[0] == "sources" && components[1].is_valid_int()) {
			
			int source_id = components[1].to_int();
			if (has_source(source_id)) {
				remove_source(source_id);
			}
			add_source(p_value, source_id);
			return true;
		} else if (components.size() == 2 && components[0] == "tile_proxies") {
			ERR_FAIL_COND_V(p_value.get_type() != Variant::ARRAY, false);
			Array a = p_value;
			ERR_FAIL_COND_V(a.size() % 2 != 0, false);
			if (components[1] == "source_level") {
				for (int i = 0; i < a.size(); i += 2) {
					set_source_level_tile_proxy(a[i], a[i + 1]);
				}