### 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)

#ifndef DISABLE_DEPRECATED
	if (p_name == "texture_offset") {
		r_ret = texture_origin;
		return true;
	}
#endif
	Vector<String> components = String(p_name).split("/", true, 2);
	if (tile_set) {
		if (components.size() == 2 && components[0].begins_with("occlusion_layer") && components[0].trim_prefix("occlusion_layer_").is_valid_int()) {
			
			int layer_index = components[0].trim_prefix("occlusion_layer_").to_int();
			ERR_FAIL_COND_V(layer_index < 0, false);
			if (layer_index >= occluders.size()) {
				return false;
			}
			if (components[1] == "polygon") {
				r_ret = get_occluder(layer_index);
				return true;
			}
		} else if (components.size() >= 2 && components[0].begins_with("physics_layer_") && components[0].trim_prefix("physics_layer_").is_valid_int()) {
			
			int layer_index = components[0].trim_prefix("physics_layer_").to_int();
			ERR_FAIL_COND_V(layer_index < 0, false);
			if (layer_index >= physics.size()) {
				return false;
			}
			if (components.size() == 2) {
				if (components[1] == "linear_velocity") {