### 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)

} else if (components[1] == "physics_material") {
				Ref<PhysicsMaterial> physics_material = p_value;
				while (index >= physics_layers.size()) {
					add_physics_layer();
				}
				set_physics_layer_physics_material(index, physics_material);
				return true;
			}
		} else if (components.size() >= 2 && components[0].begins_with("terrain_set_") && components[0].trim_prefix("terrain_set_").is_valid_int()) {
			
			int terrain_set_index = components[0].trim_prefix("terrain_set_").to_int();
			ERR_FAIL_COND_V(terrain_set_index < 0, false);
			if (components[1] == "mode") {
				ERR_FAIL_COND_V(p_value.get_type() != Variant::INT, false);
				while (terrain_set_index >= terrain_sets.size()) {
					add_terrain_set();
				}
				set_terrain_set_mode(terrain_set_index, TerrainMode(int(p_value)));
			} else if (components.size() >= 3 && components[1].begins_with("terrain_") && components[1].trim_prefix("terrain_").is_valid_int()) {
				int terrain_index = components[1].trim_prefix("terrain_").to_int();
				ERR_FAIL_COND_V(terrain_index < 0, false);
				if (components[2] == "name") {
					ERR_FAIL_COND_V(p_value.get_type() != Variant::STRING, 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_name(terrain_set_index, terrain_index, p_value);